Merriam-Webster Collegiate Dictionary - antithesis

 noun  (plural antitheses)  Etymology: Late Latin, from Greek, literally, opposition, from antitithenai to oppose, from anti- + tithenai to set — more at do  Date: 1529  1.  a.  (1) the rhetorical contrast of ideas by means of parallel arrangements of words, clauses, or sentences (as in “action, not words” or “they promised freedom and provided slavery”)  (2) opposition, contrast the ~ of prose and verse  b.  (1) the second of two opposing constituents of an ~  (2) the direct opposite her temperament is the very ~ of mine  2. the second stage of a dialectic process