mitigation has 3 distinct synonyms spread across 3 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
to act in such a way as to cause an offense to seem less serious
a partial excuse to mitigate censure; an attempt to represent an offense as less serious than it appears by showing mitigating circumstances
Synonyms, antonyms and the broader and narrower relations for mitigation come from Princeton University’s WordNet. Full sources and licences.
the action of lessening in severity or intensity
“the object being control or moderation of economic depressions”