In music notation, an accidental is a sharp, flat, natural, double sharp, double flat, or double natural sign to the left of the note-head of a particular note. It indicates that the note following it should be changed by a half step or whole step.
An accidental changes a note the same way that a sharp, flat, or natural in the key signature will, but with two significant differences. First, the change remains in effect only for the current measure. Second, the change only applies to that specific note, in that specific octave -- if a high C is changed to a C-sharp, then a middle C in the same measure remains unchanged unless given its own accidental.
A composer may repeat an accidental in parentheses as a reminder. This may be done either in the same measure to remind that the accidental is still in force, or in the following measure to remind that the key signature is again in effect.