antithesis

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Direct contrast; opposition.
• noun The direct or exact opposite.
• noun A figure of speech in which sharply contrasting ideas are juxtaposed in a balanced or parallel phrase or grammatical structure, as in "Hee for God only, shee for God in him" (John Milton).
• noun The second and contrasting part of such a juxtaposition.
• noun The second stage of the Hegelian dialectic process, representing the opposite of the thesis.

from The Century Dictionary.

• noun Opposition; contrast.
• noun That which is opposed or contrasted, as one of two opposite judgments or propositions: in this sense opposed to thesis (which see). Specifically
• noun In rhetoric, a figure consisting in bringing contrary ideas or terms into close opposition; a contrast or an opposition of words or sentiments: as, “When our vices leave us, we flatter ourselves we leave them”; “The prodigal robs his heir, the miser robs himself”; “Excess of ceremony shows want of breeding.”

from the GNU version of the Collaborative International Dictionary of English.

• noun (Rhet.) An opposition or contrast of words or sentiments occurring in the same sentence; as, “The prodigal robs his heir; the miser robs himself.” “He had covertly shot at Cromwell; he now openly aimed at the Queen.”
• noun The second of two clauses forming an antithesis.
• noun Opposition; contrast.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A proposition that is the diametric opposite of some other proposition.
• noun rhetoric A device by which two contrasting ideas are juxtaposed in parallel form.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun exact opposite
• noun the juxtaposition of contrasting words or ideas to give a feeling of balance