A pitch shifter is a sound effects unit that raises or lowers the pitch of an audio signal by a preset interval. For example, a pitch shifter set to increase the pitch by a fourth will raise each note three diatonic intervals above the notes actually played. Simple pitch shifters raise or lower the pitch by one or two octaves, while more sophisticated devices offer a range of interval alterations. Pitch shifters are included in most audio processors today.
source: Wikipedia