passionless has 1 distinct synonym spread across 1 of its 2 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 one recorded opposite is passionate.
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.
not passionate
“passionless observation of human nature”
unmoved by feeling
“he kept his emotionless objectivity and faith in the cause he served”
Synonyms, antonyms and the broader and narrower relations for passionless come from Princeton University’s WordNet. Full sources and licences.