This is the trial balloon; if we agree on this approach, a full-scale cleanup is required.

Rebased.

What is the c short for? Maybe \defnadj{foo}{bar} (as in, this is a definition of the adjective "foo" when applied to "bar"s) would be a be better mnemonic device?

"c" stands for "see", because this form of \defn adds a "see" entry. We might use this form also for non-adjective cases, e.g. \defnc{horse}{doctor} (adds "horse doctor, see doctor, horse").

Hmm, would it really be appropriate to use the same macro for both of those cases? I could imagine we might choose one day to italicize (or, say, hyperlink) only the "trivial" in "A \defnc{trivial}{class} is [...]", whereas we'd want to italicize both words in "A \defnc{horse}{doctor} is [...]". The two seem semantically different, so using different markup may be best.

In the ISO Terms and Definitions sense, the former case is

trivial
<class> trivially-copyable with a trivial default constructor

whereas the latter is

horse doctor
doctor specializing in horses

@zygoloid: Adjusted to \defnadj.