trial has 6 distinct synonyms spread across 4 of its 6 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.
the act of testing something
“in the experimental trials the amount of carbon was measured separately”
Synonyms, antonyms and the broader and narrower relations for trial come from Princeton University’s WordNet. Full sources and licences.
trying something to find out about it
“a sample for ten days free trial”
(law) the determination of a person's innocence or guilt by due process of law
“he had a fair trial and the jury found him guilty”
(sports) a preliminary competition to determine qualifications
“the trials for the semifinals began yesterday”
an annoying or frustrating or catastrophic event
“his mother-in-law's visits were a great trial for him”