'accidental' definitions:

Definition of 'accidental'

(from WordNet)
adjective
Happening by chance or unexpectedly or unintentionally ; "with an inadvertent gesture she swept the vase off the table"; "accidental poisoning"; "an accidental shooting" [syn: accidental, inadvertent]
noun
A musical notation that makes a note sharp or flat or natural although that is not part of the key signature

Synonyms of 'accidental'

From: Moby Thesaurus