flexible has 7 distinct synonyms spread across 4 of its 5 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 inflexible.
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.
capable of being changed
“flexible schedules”
able to adjust readily to different conditions
“an adaptable person”
Synonyms, antonyms and the broader and narrower relations for flexible come from Princeton University’s WordNet. Full sources and licences.
bending and snapping back readily without breaking
making or willing to make concessions
“loneliness tore through him...whenever he thought of...even the compromising Louis du Tillet”