functional has 7 distinct synonyms spread across 2 of its 6 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 nonfunctional, organic.
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.
designed for or capable of a particular function or use
“a style of writing in which every word is functional”
involving or affecting function rather than physiology
“functional deafness”
fit or ready for use or service
“the toaster was still functional even after being dropped”
2 of the 6 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for functional.
Synonyms, antonyms and the broader and narrower relations for functional come from Princeton University’s WordNet. Full sources and licences.