cap has 13 distinct synonyms spread across 7 of its 11 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 top (as for a bottle)
a mechanical or electrical explosive device or a small amount of explosive; can be used to initiate the reaction of a disrupting explosive
something serving as a cover or protection
a fruiting structure resembling an umbrella or a cone that forms the top of a stalked fleshy fungus such as a mushroom
an upper limit on what is allowed
“he put a ceiling on the number of women who worked for him”
(dentistry) dental appliance consisting of an artificial crown for a broken or decayed tooth
“tomorrow my dentist will fit me for a crown”
restrict the number or amount of
“We had to cap the number of people we can accept into our club”
Synonyms, antonyms and the broader and narrower relations for cap come from Princeton University’s WordNet. Full sources and licences.