Accidental

The sign indicating momentary departure from the current key signature by raising or lowering a note by means of a sharp, a flat, or a natural. It holds good throughout the measure unless contradicted, and where it occurs attached to the last note of the measure and this note is tied to a note in the next measure, it holds good for that latter note also. In some 20th century music, any accidental that occurs is understood to affect only the note before which it is placed, as was often the case with music from the medieval period to the 17th century.