jam has 25 distinct synonyms spread across 8 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.
push down forcibly
“The driver jammed the brake pedal to the floor”
get stuck and immobilized
“the mechanism jammed”
preserve of crushed fruit
Synonyms, antonyms and the broader and narrower relations for jam come from Princeton University’s WordNet. Full sources and licences.
deliberate radiation or reflection of electromagnetic energy for the purpose of disrupting enemy use of electronic devices or systems