antonym

But, oddly enough, the eyes were a total and surprising antonym of this smiling mouth.

In strict usage, daily is the antonym of nightly as diurnal is of nocturnal.

By and by, which was once a synonym, has become an antonym of immediately, meaning at some (perhaps remote) future time.

Logically the other side of white is not white, while the antonym is the extreme black.

The direct antonym of cause is effect, while that of antecedent is consequent.