Open English Wordnet
Nouns
(n)accidentala musical notation that makes a note sharp or flat or natural although that is not part of the key signature
Adjectives
(s)accidental, inadvertenthappening by chance or unexpectedly or unintentionally“with an inadvertent gesture she swept the vase off the table”, “accidental poisoning”, “an accidental shooting”
(s)incidental, nonessential, accidentalnot of prime or central importance“nonessential to the integral meanings of poetry” (Pubs.MLA), “the character's motives remain accidental to the plot”