antithesis (n.) Look up antithesis at Dictionary.com
1520s, "opposition, contrast," originally in rhetoric, "the bringing of contrary ideas or terms in close opposition;" 1530s as "that which is in (rhetorical) opposition or contrast," from Late Latin antithesis, from Greek antithesis "opposition, resistance," literally "a placing against," also a term in logic and rhetoric, noun of action from antitithenai "to set against, oppose," a term in logic, from anti "against" (see anti-) + tithenai "to put, place," from reduplicated form of PIE root *dhe- "to set, put."

The extended sense of "direct or striking opposition" is from 1630s; as "that which is the direct opposite" from 1831.