Is it ensure, insure, or assure?
late 14c., "non-essential," from Old French accidentel or directly from Medieval Latin accidentalis, from Latin accidentem (see accident). Meaning "outside the normal course of nature" is from early 15c.; that of "coming by chance" is from 1570s.
late 14c., "non-essential quality," from accidental (adj.). The musical sense is from 1868.
in music, sign placed immediately to the left of (or above) a note to show that the note must be changed in pitch. A sharp () raises a note by a semitone; a flat () lowers it by a semitone; a natural () restores it to the original pitch. Double sharps () and double flats () indicate that the note is raised or lowered by two semitones. Sharps or flats that are placed at the beginning of a musical staff, called a key signature, indicate the tonality, or key, of the music and are not considered accidentals.