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The Collaborative International Dictionary
Counterterm

Counterterm \Coun"ter*term`\ (-t?rm`), n. A term or word which is the opposite of, or antithesis to, another; an antonym; -- the opposite of synonym; as, ``foe'' is the counterterm of ``friend''.
--C. J. Smith.

Wiktionary
counterterm

n. 1 antithesis 2 antonym