Q: hey bill, is an accidental any one of the black notes on the keyboard, or is it like a note that is not in the key that you're playing in?
A: in written music, it is any note outside of the key signature. specifically, it is the flat or sharp or natural symbol that you have to use in the middle of the music, which was not included in the key signature. the name 'accidental' is deceptive because it's not really an accident