Antithetical and antithesis come from Greek antitithenai ("to oppose"). The oldest sense of antithesis refers to a language pattern that contrasts parallel ideas, as in "action, not words" or "they promised freedom and provided slavery," and antithetical originally referred to anything that was marked by such antithesis. For example, you could say "The phrase 'action, not words' is an antithetical construction." It is more common, however, for antithesis to mean "the exact opposite" and for antithetical to mean "directly opposite."