impact has 9 distinct synonyms spread across 4 of its 6 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.
a forceful consequence; a strong effect
“the book had an important impact on my thinking”
influencing strongly
“they resented the impingement of American values on European culture”
press or wedge together; pack together
Synonyms, antonyms and the broader and narrower relations for impact come from Princeton University’s WordNet. Full sources and licences.