In music, an accidental refers to any note that does not belong in a given key as indicated by the most recent key signature. Accidentals are displayed using sharps (♯), flats (♭), and naturals (♮).
Accidentals don’t apply beyond the next bar line, so if you have an E♮ in the key of B♭ major, for instance, it will go back to being E♭ in the next measure. Composers and editors often place a courtesy accidental in the following measure to remind the performer what the note should be given the key signature. Courtesy accidentals are generally surrounded by parentheses.