One case shown to make a general thing graspable. An example teaches and can also mislead, since one case is never the rule.
Latin exemplum (a sample, a pattern), from eximere (to take out), ex- plus emere (to take, to buy).
Its root shares a root with redeem and premium: emere meant to take, even to buy. An example is a single case taken out of a larger stock to stand for it, useful exactly as long as no one mistakes the piece taken for the whole stock it came from.