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