inactive has 6 distinct synonyms spread across 4 of its 10 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 active.
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.
(pathology) not progressing or increasing; or progressing slowly
(military) not involved in military operations
not exerting influence or change
lacking in energy or will
“Much benevolence of the passive order may be traced to a disinclination to inflict pain upon oneself”
lacking activity; lying idle or unused
“an inactive mine”
not engaged in full-time work
“inactive reserve”
not active physically or mentally
“illness forced him to live an inactive life”
1 of the 10 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for inactive.
Synonyms, antonyms and the broader and narrower relations for inactive come from Princeton University’s WordNet. Full sources and licences.