1520s, 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 place," from PIE root *dhe- "to put, to do" (see factitious).
a figure of speech in which irreconcilable opposites or strongly contrasting ideas are placed in sharp juxtaposition and sustained tension, as in the saying "Art is long, and Time is fleeting."