transient has 6 distinct synonyms spread across 2 of its 4 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.
one who stays for only a short time
“transient laborers”
(physics) a short-lived oscillation in a system caused by a sudden change of voltage or current or load
Synonyms, antonyms and the broader and narrower relations for transient come from Princeton University’s WordNet. Full sources and licences.
of a mental act; causing effects outside the mind
lasting a very short time
“the ephemeral joys of childhood”