1. a thesis, idea, or proposition that is opposite to or contradicts another.

2. in philosophy, the second stage of a dialectical process based on proposition, contradiction, and the reconciliation of these (thesis, antithesis, and synthesis). The term is often associated with German philosopher Georg Wilhelm Friedrich Hegel (1770–1831) and his theory of the dialectical development of ideas, although it was not a term he used in his analysis. See also dialectical materialism. —antithetical adj.