corps

Civilian life borrowed the word from the army formation, and it still suggests people who act as one body under a shared task. It is most at home behind a word that names the field, as in the diplomatic corps or the White House press corps, where it means everyone in that line of work at once.

In the chain of formations it sits above the division and below the army: an army splits into corps, and a corps into divisions. The word came from French corps d'armée, “army body”, from Latin corpus, so English holds the same root three times over, in corps, corpse and corpus. The plural is spelled like the singular: one corps, several corps.

Here the -s is an ordinary plural, added to corp, the short form of corporation. That makes it a perfect lookalike of the word for an organized group, whose spelling never changes; talk of business or markets usually settles which is meant, and corporations is the safer plural when nothing does.