informally has 2 distinct synonyms 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 formally.
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.
without formality
“he visited us informally”
with the use of colloquial expressions
“this building is colloquially referred to as The Barn”
Synonyms, antonyms and the broader and narrower relations for informally come from Princeton University’s WordNet. Full sources and licences.