antĭthĕsis, is, f., = ἀντίθεσις, a gram. ng.,
the putting of one letter for another (e. g. olli for illi, impete for impetu), Charis. p. 249 P.; Diom. p. 437 P.
·noun Opposition; contrast. II. Antithesis ·noun The second of two clauses forming an antithesis. ...
Webster's Dictionary of the English Language