Used in philosophical, logical, or rhetorical contexts to mean a proposition that opposes a given thesis. Also used more broadly for a direct opposite or contrast.
from German Antithese
In Hegelian dialectics, a synthesis arises from a thesis and an antithesis.
彼の意見は、私の考えに対するアンチテーゼだった。
His opinion was the antithesis of my idea.