ac·ci·den·tal / ˌaksiˈdentl/ • adj. 1. happening by chance, unintentionally, or unexpectedly.2. incidental; subsidiary.3. Philos. (in Aristotelian thought) relating to or denoting properties that are not essential to a thing's nature.• n. 1. Mus. a sign indicating a momentary departure from the key signature by raising or lowering a note.2. Ornithol. another term for vagrant.DERIVATIVES: ac·ci·den·tal·ly adv.

accidental. The sign indicating momentary departure from the key signature by the raising or lowering of a note by means of a sharp, flat, natural, etc. It holds good throughout the measure (bar) unless contradicted, and where it occurs attached to the last note of the measure and this note is tied to a note in the next measure, it holds good for that latter note also. In some 20th-cent. mus. any accidental which occurs is understood to affect only the note before which it is placed, as was also often the case with mus. from the medieval period to the 17th cent.