Open English Wordnet
Adverbs
(r)by chance, accidentally, circumstantially, unexpectedlywithout advance planning“they met accidentally”
(r)incidentally, accidentallyof a minor or subordinate nature“these magnificent achievements were only incidentally influenced by Oriental models”
(r)unintentionally, accidentallywithout intention; in an unintentional manner“she hit him unintentionally”