formal has 8 distinct synonyms spread across 4 of its 8 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 informal.
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.
being in accord with established forms and conventions and requirements (as e.g. of formal dress)
“pay one's formal respects”
(of spoken and written language) adhering to traditional standards of correctness and without casual, contracted, and colloquial forms
“the paper was written in formal English”
represented in simplified or symbolic form
a lavish dance requiring formal attire
2 of the 8 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 formal.
Synonyms, antonyms and the broader and narrower relations for formal come from Princeton University’s WordNet. Full sources and licences.