adjective

1. happening by chance; fortuitous
2. belonging but not essential; attributive; incidental
3. Music of an accidental

Etymology: ME < LL accidentalis: see accident

noun

1. a nonessential quality or feature
2. Music
   1. a sign, as a sharp, flat, or natural, placed before a note to show a change of pitch from that indicated by the key signature
   2. the tone indicated by such a sign