loser has 4 distinct synonyms spread across 2 of its 3 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. Its 2 recorded opposites are winner, achiever.
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 contestant who loses the contest
a person with a record of failing; someone who loses consistently
Synonyms, antonyms and the broader and narrower relations for loser come from Princeton University’s WordNet. Full sources and licences.