pessimism has no recorded synonym. 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 one recorded opposite is optimism.
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 general disposition to look on the dark side and to expect the worst in all things
Synonyms, antonyms and the broader and narrower relations for pessimism come from Princeton University’s WordNet. Full sources and licences.