sonic has 1 distinct synonym spread across 1 of its 2 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. Its 2 recorded opposites are subsonic, supersonic.
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.
(of speed) having or caused by speed approximately equal to that of sound in air at sea level
“a sonic boom”
1 of the 2 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for sonic.
Synonyms, antonyms and the broader and narrower relations for sonic come from Princeton University’s WordNet. Full sources and licences.