Phasing

Phasing is an audio effect in which a sound combines with an out-of-phase replica of itself to produce a comb filter effect. A phaser is an electronic device used to produce this effect. See also flanging.

The compositional technique phasing discovered by composer Steve Reich is that while the same part is played on two instruments, one instrumentalist keeps playing in steady tempo, while the other gradually moves ahead until it becomes back in phase (the rhythmic equivalent of cycling through the phase of two waveforms).