Definition of “antithesis” - Learner’s Dictionary

antithesis

noun [ C ] uk us /ænˈtɪθəsɪs/ (plural antitheses /ænˈtɪθəsiːz/) formal

the exact opposite:

[ usually singular ] She is slim and shy - the antithesis of her sister.

(Definition of “antithesis” from the Cambridge Learner's Dictionary © Cambridge University Press)