A note that falls outside the key signature. For example, a C sharp occurring in a melody written in the key of C would be an accidental; however, it would not be an accidental in a melody written in A major. Sharps, flats, naturals, double sharps, and double flats are the accidentals commonly used.
Modern composers are not always strictly logical in their use of accidentals. CHARLES IVES scolded a copyist for trying to correct a seemingly wrong accidental by scribbling in the margin of his manuscript: "Please don't correct the wrong notes. The wrong notes are right." But NIKOLAI RIMSKY-KORSAKOV took offense when his Russian copyist put a question mark over an obviously wrong accidental: "I am not somebody like RICHARD STRAUSS or DEBUSSY, to put in wrong notes deliberately. So fix it!"