stemmer has 3 distinct synonyms spread across 2 of its 5 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 worker who makes or applies stems for artificial flowers
an algorithm for removing inflectional and derivational endings in order to reduce word forms to a common stem
a miner's tamping bar for ramming packing in over a blasting charge
a device for removing stems from fruit (as from grapes or apples)
Synonyms, antonyms and the broader and narrower relations for stemmer come from Princeton University’s WordNet. Full sources and licences.