accidental

In music notation, a sharp (♯), flat (♭), or natural (♮) sign preceding a note which changes the pitch of the note by a semitone up (♯), down (♭), or back to normal (♮).

Less common is the double sharp (♯♯), which raises a note by two semitones, and the double flat (♭♭), which lowers a note by two semitones (there is an example of a triple flat in one of French composer Alkan's piano works).

Once an accidental appears in a bar, it applies to all the other notes of the same pitch that follow until the end of the bar.