Definition of: accidental

(ak′sə·den′təl) adjective
1. Happening or coming by chance or without design; casual; fortuitous; taking place unexpectedly, unintentionally, or out of the usual course.
2. Non-essential; subordinate; incidental: said of any attribute or feature not entering into the very nature of a thing.
3. Music Pertaining to or indicating a sharp, natural, flat, etc., elsewhere than in the signature.
—noun
1. A casual, incidental, or non-essential feature or property.
2. Music A sharp, flat, or natural elsewhere than in the signature. There are five accidentals or signs of chromatic alteration to show that the notes to which they are applied have to be raised or lowered a semitone or a tone: the sharp, double-sharp, flat, double-flat, and the natural.
—ac′ci·den′tal·ness noun Synonyms (adj.): chance, uncontemplated, undesigned, unexpected, unforeseen, unintended, unpremediated. See INCIDENTAL. Antonyms: calculated, contemplated, deliberate, designed, intended, planned, preconcerted, purposed, studied.