Accidental a. : Happening by chance, or unexpectedly; taking place not according to the usual course of things; casual; fortuitous; as, an accidental visit.

Accidental a. : Nonessential; not necessary belonging; incidental; as, are accidental to a play.

Accidental n. : A property which is not essential; a nonessential; anything happening accidentally.

Accidental n. : Those fortuitous effects produced by luminous rays falling on certain objects so that some parts stand forth in abnormal brightness and other parts are cast into a deep shadow.

Accidental n. : A sharp, flat, or natural, occurring not at the commencement of a piece of music as the signature, but before a particular note.

Example:
How do you know if the "Accidental" touch is a flirting touch and not actually Accidental?

Taxonomy
pitch, measure, the accidentals, key signature, semitone, raises, music, hexachord, square, double sharp, sharps, octave, naturals, flat sign, notation, free encyclopedia, flats, music notation.

Defination from the web:
accidental is a note whose pitch (or pitch class) is not a member of a scale or mode indicated by the most recently applied key signature. ... An accidental sign raises or lowers the following note from its normal pitch, usually by a semitone, although microtonal music may use " ...