jacket has 4 distinct synonyms spread across 1 of its 7 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 short coat
an outer wrapping or casing
“phonograph records were sold in cardboard jackets”
(dentistry) dental appliance consisting of an artificial crown for a broken or decayed tooth
“tomorrow my dentist will fit me for a crown”
the tough metal shell casing for certain kinds of ammunition
Synonyms, antonyms and the broader and narrower relations for jacket come from Princeton University’s WordNet. Full sources and licences.