Meaning of accidental

(adj) happening by chance or unexpectedly or unintentionally; "with an inadvertent gesture she swept the vase off the table"; "accidental poisoning"; "an accidental shooting"

not of prime or central importance; "nonessential to the integral meanings of poetry"- Pubs.MLA; "the character's motives remain accidental to the plot"

(noun) a musical notation that makes a note sharp or flat or natural although that is not part of the key signature