The One Meaning Illusion

In response to my recent post titled "Uber driver with gun apparently stops would-be mass shooter; have civilians stopped such mass shootings before?," a commenter wrote,

I hope someone has pointed out that "ordinary" citizens and police ARE civilians. "Civilian" distinguishes military from non-military; it does not distinguish government employees from other people.

This reminded me of a common mistake that I've seen people fall into: people know one definition for a word, and just casually assume that some other usage must be wrong, because it's inconsistent with that definition. Yet of course English words often have multiple definitions. "Civilian," for instance, sometimes mean "not a member of the military," but sometimes it means "not a police officer" (or perhaps neither a police officer nor a military member).

If you look in the dictionary, you can see the various definitions, sometimes separate and sometimes as part of one entry; for instance, the American Heritage Dictionary defines "civilian" as "A person who is not an active member of the military, the police, or a belligerent group in a conflict." Dictionary.com gives a similar definition. But if your first reaction is just to think "that's not what 'civilian' means, since I know the meaning of 'civilian'"—with an implied emphasis on the "the"—then you might not even bother to check the dictionary. And then you might make claims about a supposed error that proves to be no error at all.

Incidentally, it seems to me that the alternative meanings of "civilian" stem from its being an oppositional term: a term that is used to distinguished one class of person or thing from another class, which is often drawn from context. "Layman" is a classic example—when you're talking about religious matters, a layman is someone who isn't clergy; when you're talking about medicine, a layman is someone who isn't a doctor; when you're talking about law, a layman is someone who isn't a lawyer. "Cash" is another. When you say that you paid cash at the supermarket, you probably mean you paid with bills rather than with a check or a credit card. When you say that you paid cash for a car, you probably mean you paid with a check rather than by getting credit. So it's especially unsound, I think, to assume that such context-dependent terms have a fixed, context-independent meaning. But the One Meaning Illusions arises for other words as well.