accidentally has 5 distinct synonyms spread across 3 of its 3 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 deliberately.
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.
of a minor or subordinate nature
“these magnificent achievements were only incidentally influenced by Oriental models”
without intention; in an unintentional manner
Synonyms, antonyms and the broader and narrower relations for accidentally come from Princeton University’s WordNet. Full sources and licences.
“she hit him unintentionally”