A titmouse is not a mouse. Buckwheat is not wheat. As Voltaire pointed out, the Holy Roman Empire was in no way holy, nor Roman, nor an empire. Additional examples of weird terminology can be found in section 2.
It is important to keep in mind the simple rule:
Most names should be considered idiomatic expressions. As such, they should not be taken literally.
Names exist for a reason. Many of the things we deal with on a daily basis are not easy to describe in detail; it would take several sentences or even several pages for a full description. A fully-descriptive name would be far too long to be useful. Therefore we have dictionaries and encyclopedias, where we can look up the name to find a detailed description.
It must be emphasized that ideas are primary and fundamental, while terminology is tertiary. Terminology is important only insofar as it helps us think about and talk about the ideas.
We get into trouble when the name is ambiguous (the same name applied to multiple distinct ideas) ... or when the name appears to be descriptive but is misleading.
|“Inaction” is the opposite of “action”. As a rule, in English, the prefix “in–” negates the meaning of the word that follows. Many, many words follow this rule.||“Inflammable” means the same thing as “flammable”. The prefix “in–” does not negate “flammable”, so this is an exception to the usual rule. If you follow the rule, you will get the meaning diametrically wrong.|
It has long been known that the word “inflammable” is misleading to a dramatically dangerous degree. Therefore there was a concerted effort get rid of that word entirely, and to standardize on “flammable”. However, related words such as “inflammatory” are still extremely common.
Richard Feynman was fond of pointing out that knowing the name of a thing is not equivalent to understanding a thing. In reference 1 he wrote:
The next day, Monday, we were playing in the fields and this boy said to me, “See that bird standing on the stump there? What’s the name of it?”
I said, “I haven’t got the slightest idea.”
He said, “It’s a brown-throated thrush. Your father doesn’t teach you much about science.”
I smiled to myself, because my father had already taught me that [the name] doesn’t tell me anything about the bird. He taught me “See that bird? It’s a brown-throated thrush, but in Germany it’s called a halsenflugel, and in Chinese they call it a chung ling and even if you know all those names for it, you still know nothing about the bird–you only know something about people; what they call that bird. Now that thrush sings, and teaches its young to fly, and flies so many miles away during the summer across the country, and nobody knows how it finds its way,” and so forth. There is a difference between the name of the thing and what goes on.
The result of this is that I cannot remember anybody’s name, and when people discuss physics with me they often are exasperated when they say “the Fitz-Cronin effect,” and I ask “What is the effect?” and I can’t remember the name.
However, the young Feynman made a mistake by over-reacting. Knowing the name of the bird does in fact tell you something. First of all, there is a good chance that a brown-throated thrush is related to other thrushes, and that tells you quite a lot, if you have studied other thrushes. And even if this is the first thrush you’ve ever seen, or even the first bird, knowing that it is a brown-throated thrush allows you to look it up in reference books, and thereby find out enormous amounts of information.
Names are a tool. As such, they can be used wisely or unwisely:
|Like any tool, names can be abused.||Just because a tool can be abused does not mean you are obliged to abuse it.|
In general, you should not expect names to be descriptive ... and even if you have a chance to give something a descriptive name, please do not to overboard in that direction. For example, when naming the variables and the subroutines in a computer program, oftentimes it would take several sentences or several paragraphs to fully describe what the thing does ... and that’s too long for a convenient name. It is better to give it a short name and then write into the documentation a legend that says what the name means. For more on this, see reference 2.
Oftentimes, the meaning of a noun or verb is heavily modified by the surrounding words, even if the core meaning remains more-or-less the same. For example, consider
Here’s a more complex example. Consider the constrast:
X divided by Y has only one meaning:
X divided into Y has multiple
meanings, depending on context:
Note that in example (a) we have 3 groups of 7, while in example (b) we have 7 groups of 3. Meanwhile, in examples (a) and (b) we have X/Y=7, while in example (c) the fraction is upside down: Y/X=7.
Overall, inconsistent terminology gives rise to multiple problems.
A minor problem is that if somebody asks you what a given word means, it is impossible to give a concise answer. It all depends on how the word is used.
A much more serious problem is that if you know what a word means in one context, you might assume it has the same meaning in another context. This can lead to serious mistakes. It’s easy to have high confidence in the wrong answer, which makes things even worse.
If you’re lucky, the intended meaning can be figured out by sufficiently close reading of the sentence; the examples in this section are in this category. However, things can get much worse than that: Sometimes the meaning depends on the broader context, and sometimes it’s just hopelessly ambiguous.
The following list gives some examples where the name of the thing provides a conspicuously poor description of the thing ... or where the same word has multiple wildly-divergent meanings.
It should go without saying that this list is nowhere near complete. These few examples should suffice to make the point that names are not the same as descriptions, and you should not read too much into a name.
|In ordinary English, if somebody says A and B and C and D are on top of the table, as we increase the number of terms in the and-expression, the set of objects gets larger.||In Boolean algebra and formal logic, the and operator denotes the intersection of sets. If somebody speaks of A and B and C and D, as we increase the number of factors in the and-expression, the number of objects gets smaller (or stays the same).|
|In mathematics, A = B means exactly the same thing as B = A. It’s completely symmetrical, by definition.||In many computer languages such as C++, fortran, or basic, the assignment statement A = B means something wildly different from B = A. There’s nothing symmetrical about it.|
There are some rather serious misconceptions about this, having to do with equivalence versus causation, in connection with the laws of physics, as discussed in reference 3.
The Algol computer language uses the “:=” symbol for assignment statements, which has an appropriately asymmetrical appearance. Meanwhile, the Macsyma language (like its clone, Maxima) uses the “:=” symbol for defining functions. Oddly enough, it uses a simple “:” for assignment statements. This symbol has a less-than-ideal symmetrical shape, but at least it does not directly conflict with the mathematical “=” sign.
|Sometimes the term “in general” refers to the most-usual, most-prevalent case, allowing for possible exceptions.||Sometimes the term “in general” refers to something that is absolutely, universally true.|
Because of the ambiguity, I prefer to avoid the term (except in negative constructions). Alternatives include saying “as a decent rule of thumb” (if that’s what I mean) or saying “in all generality” (if that’s what I mean). Very few things are true in all generality; even the most basic theorems are subject to important provisos, restrictions, and assumptions.
|As mentioned in section 1, the prefix “in−” is widely used to form opposites. For example, “inaction” is the opposite of “action”, and “invertebrate” is the opposite of “vertebrate”.||The word “inflammable” means the same thing as “flammable”. This does not conform to the usual pattern. It is remarkably and dangerously misleading.|
|In swimming, one lap is one length of the pool. It takes two laps to get back to the starting place.||On a running track, one lap is once around the track, back to the starting place.|
|The races were sanctioned by the state council. (Meaning approved.)||Two of the runners were sanctioned by the state council. (Meaning disapproved and penalized.)|
|We speak of the “alkali metals” as being disjoint from the “alkali earths”.||The alkali earths are perfectly good metals.|
Constructive suggestion: It helps to avoid the term “acidity” as much as possible. If you mean low pH, say low pH.
The whole notion of “alkalinity” (aka “total alkalinity”) always struck me as unsophisticated and underspecified. Buffer behavior is complicated, not easily characterized by a single number. Sometimes you care about the local height, and sometimes you care about the area under the curve.
|Astronomers use the name “metals” to apply to any elements other than hydrogen and helium.||Chemists, metallurgists, and ordinary folks share a notion of “metal” that is very much narrower than the astronomers’ notion.|
|In the context of an electrical harmonic oscillator, such as one standing-wave mode of the electromagnetic field, we say that the Nth energy eigenstate has N “photons” in it. The operator a†a is the photon-number operator. These photons do not propagate at the speed of light; indeed they do not propagate at all. They are standing waves ... or in the case of the simple harmonic oscillator, not really waves at all.||In the context of a propagating wave, a “photon” is a wavepacket, typically a Gaussian wavepacket, with some not-too-large spread in position and also some not-too-large spread in momentum.|
|We speak of “oxygen” (from the Greek, meaning literally “acid former’) and “halogens” (literally “salt formers”).||For every salt there is a corresponding acid, and for every acid there is a corresponding series of salts. So how can oxygen and halogen be disjoint notions? Every acid-former should also be a salt-former, and vice versa.|
|We speak of the “rare earths”.||They are not particularly rare. For example, cerium is slightly more abundant than copper.|
|The noun “day” can refer to the hours of daylight (roughly a 12-hour day).||It can also refer to the complete 24-hour cycle.|
|Ditto for the adjectives “daily” and “diurnal”. As far as I can tell the ambiguity has existed for centuries, dating back to the Latin dies and diurnus.|
|Bimonthly “usually” means once every two months.||According to the American Heritage Dictionary, it can also mean twice per month. On the other hand, other dictionaries label the latter definition as rare or erroneous.|
|The same ambiguity arises for other words such as biweekly.|
|We speak of cooking things “in the microwave”.||That refers to a microwave oven, which is an oven, not a microwave (nor, indeed, a wave of any kind).|
|The microwaves in such an oven have a wavelength that is not microscopic, is not micron-sized, and is in fact much longer than the wavelength of the waves that do the cooking in a plain old broiler.|
|Sometimes “X-ray” refers to a particular part of the electromagnetic spectrum.||Sometimes “X-ray” refers to an image made using this part of the spectrum.|
|I recently cooked some “French fries”. I cooked them in the oven, in accordance with instructions on the package.||That resulted in “French fries” that had never been fried and had never been anywhere near France.|
|Wish-Bone sells something called “French dressing”.||It doesn’t come from France, and does not resemble anything commonly served in France.|
None of these four meanings have much in common nowadays. They are all rather distantly related to the original root, namely instrument or mechanism.
Neither the chemists nor the physicians should be telling the farmers how to define farming-related terms ... and vice versa, in all combinations.
|In mechanics, we speak of “kinetic energy” as being disjoint from “potential energy”.||In thermodynamics, we speak of the “chemical potential”. A large part of the chemical potential consists of kinetic energy.|
|In physics, conservation of momentum means that the amount of momentum in a region cannot change except insofar as it flows across the boundary. See equation 1. Energy, momentum, and electrical charge are always strictly conserved.||In a non-technical context, conservation means something very different, namely avoiding waste. For example: conservation of endangered wildlife.|
|In physics, the notion of energy is fundamental and very important. It is a state function, i.e. a function of the state of a give system or parcel.||The homespun term “energy” refers to something else. It is not the physics energy, or even the thermodynamic free energy. Roughly speaking, it is the amount of thermodynamically available energy. It is not a function of state, since it depends on the parcel’s surroundings.|
|Along with item 31, this shows that in the expression “conservation of energy”, both the word conservation and the word energy have dramatically different meanings, depending on whether you are speaking in physics terms or homespun terms.|
|We speak of the “free energy”.||Although it has dimensions of energy, it is not the actual energy.|
|Also, it’s not free, since you usually have to pay for it.|
|By itself, the word radio brings to mind electromagnetism in the kHz to MHz part of the spectrum, where the energy is 5×10−11 to 5×10−7 eV per photon.||The term radioactivity, which comes from the same root, brings to mind photons and particles with energy above 105 eV per particle.|
|So radioactivity is separated from radio by 12 orders of magnitude.|
|The term “bedlam” refers to uproar and confusion.||The word is derived from “Bethlehem”, which carries no such meaning.|
|In molecular spectroscopy we speak of “internal conversion”.||The conversion is not usually internal to the molecule.|
|Originally, “elastic” meant capable of returning to its original size and shape after being stretched. This meaning is still common in technical meanings such as the elastic limit of a spring, and in many nontechnical meanings such as an elastic band in clothing. In this sense, rubber is far more elastic than steel.||In physics there is another, much narrower meaning: we speak of an inelastic collision between two soft rubber balls, even if the two balls return to their original shape. In this sense, steel has a far greater coefficient of elasticity than rubber.|
|In the printing trades, and in the physics lab, the colors red, green, and blue are very different from the colors cyan, magenta, and yellow.||In, say, a clothing store, cyan is considered a shade of blue. If you want a cyan-colored shirt, you should ask for blue (or bright blue); if you ask for cyan the clerk probably won’t understand you. If you want a technically blue-colored shirt, you might ask for deep blue.|
|The SI “mole” is defined as the base unit for “amount of substance”. This is a subtle, highly abstract notion. It is much older than atomic theory, and does not depend counting particles of any kind.||Nowadays a lot of people define “mole” in terms of Avogadro’s number. A mole is a number, like a dozen (only larger).|
|For all practical purposes, a mole is equal to the number of 12C atoms (or the “amount of substance”) in twelve grams of 12C. However, twelve kilograms would have made SI more consistent. One occasionally sees definitions of the terms gram-mole and kilogram-mole, but these are not SI terms and are vanishingly rare in practice.|
|In some situations, adiabatic means fast enough ... so that there are no appreciable heat leaks through the boundary.||In some situations, adiabatic means slow enough or gentle enough ... so that there is a one-to-one correspondence between initial states and final states, with no change in occupation numbers. This is the opposite of the “sudden” approximation. See reference 4.|
|In thermodynamics, experts use at least four or five mutually-inconsistent reasonable technical definitions of “heat”, each of which has its advantages and disadvantages. See reference 5.||Four or five different reasonable technical definitions is bad enough, but there are also innumerable less-reasonable, non-technical, and/or metaphorical uses of the word.|
|Rock candy is not made of rock.||Rock wool is not made of wool.|
|Milk of magnesia is not made of milk.||Chocolate turtles are not made of turtles.|
|This goes to show that the English language’s rules for forming appositives are rather loose.|
|In the terms “frequency spectrum” and “mass spectrometer”, frequency and mass (respectively) are the abscissa of the spectrum.||In the terms “power spectrum” and “emission spectrum”, power and emission (respectively) are the ordinate of the spectrum.|
|Sometimes “spin zero” means s=0, which is related to the eigenvalue of the S2 operator.||Sometimes “spin zero” means ms=0, which is the eigenvalue of the Sz operator.|
|It is not hard to construct sentences which use the word “spin” with two different meanings in the same sentence. It is not hard to get thoughtful experts to accept such sentences at face value, even after being warned that a trick question is coming.|
|We speak of electrons “flowing” in a wire. Also, since electrons carry charge, we speak of charge “flowing” in a wire. Charge density appears on the LHS of equation 1. “Flow” means that charge is being carried from place to place.||We speak of current “flowing” in a wire. Current density appears on the RHS of equation 1. “Flow” does not mean that current is being carried from place to place.|
The equation for continuity of flow is:
|When drawing the vector that represents an electric dipole moment, physicists, mathematicians, and some (but not all!) chemists draw it in a way that is consistent with the orientation of position vectors.||Some chemistry books (especially at the introductory level) draw the arrow the other way. This makes no sense, but they do it anyway. See reference 7 for details.|
|Suppose we have two parallel plates, with a charge Q/2 on each of them. We say there is a charge Q on the pair of plates, referring to the actual total charge.||Suppose we have a parallel-plate capacitor, with a charge +Q on one plate and a charge −Q on the other plate. It is conventional (but unwise) to say there is a “charge” Q on the capacitor.|
|There are innumerable intermediate cases.|
|I strongly recommend reserving the term “charge” for the real total charge.||Constructive suggestion: We can use the term gorge to represent the capacitor situation. We can speak of gorging and disgorging the capacitor. See reference 8.|
|The term “linear equation” covers things like y = m x + b.||The term “linear transformation” covers the mapping from x to m x but does not include the mapping from x to m x + b (unless by some miracle b is identically zero).|
To avoid the inconsistency, the easiest thing is to avoid the term “linear” entirely. You can refer to y = m x + b as an affine relationship and refer to y = m x as a proportionality relationship.
In other words, you might expect sin−1(x) to be the multiplicative inverse of sin(x), but instead it is more-or-less universally interpreted to mean the functional inverse, i.e. the inverse function.
Similar words apply to other trigonometric functions (cosine, tangent, cotangent, et cetera).
|When a switch is closed, electrical current can flow.||When a valve is closed, no fluid can flow.|
|In mathematics, the term “field” could refer to a vector field.||Or it could refer to a Galois field.|
For more examples, see reference 9. It’s hard to see what these different algebras have in common. In particular, it’s hard to predict what sorts of mathematical structures will be called algebras and which won’t.
Even in the context of high-school algebra, the word has two different meanings: Fundamentally, algebra is a language for expressing rules, patterns, and generalizations. However, when most people hear the word “algebra” they think of solving equations to find x.
|In mathematics, an element y of a group is called “primitive” if every element of the group is equal to some power of y. For example, in the group of integers mod 7, the element 3 is primitive, while the element 2 is not.||Let F be the field of polynomials over some base field G. An element P of F is called “primitive” if every element of the field (F mod P) is equal to some power of the monomial x. For example, x2+x+1 is primitive in the field of polynomials over GF, but it is not primitive in the field of polynomials over GF.|
|This is demonstrably a source of real (not hypothetical) confusion. A well-known cryptography book quoted one definition of “primitive” in a context where the other definition was required.|
|The term “generator” is also ambiguous. According to one definition, the element y in item 58 is called a “generator” of the group.||Let F be the field of polynomials over some base field G. Let P be an irreducible element of F. Then P is called the “generator” of the field (F mod P).|
|(IMHO it would be good to do away with this usage, and instead call P the modulus.)|
|“Event” in spacetime, in physics.||“Event” in statistics.|
|“Sample” in chemistry.||“Sample” in statistics. See reference 10.|
|“Gravity” in the sense of framative gravity.||“Gravity” in the sense of massogenic gravity. See reference 11.|
|“Acceleration” (the vector).||“Acceleration” (the scalar). See reference 12.|
|Textbooks commonly define «power» as the rate of doing mechanical work.||In practice, «power» is used much more broadly. I get almost twice as many hits from googling “watts of thermal power” than from “watts of mechanical power”.|
|Some argue that any transfer or transformation of energy should count as «power» ... but I’m not sure advection should count. For example, consider a slow leak in the gas tank of a car. Does that count as «power»? I’ve pretty much given up. I consider this one of the supposedly technical terms that doesn’t have any clear meaning. Anybody who wants to use it should explicitly specify the intended meaning.|