Double tracking, also called doubling, may refer to any of the following:
A special effect that simulates two voices or instruments playing together, created by combining a signal with a duplicate signal delayed by 15 to 35 milliseconds.
A special effect used to thicken a sound by playing two identical signals together.
Recording a vocal or instrumental on one track and then re-recording the vocal or instrumental (in separate performance) on a second track (sometimes called re-tracking). The idea is that when the two tracks are played simultaneously, minor differences in timing or tone combine to make a thicker sound.