A type of audio effect very similar to a flanger. Phasers have a characteristic 'swoop' sound that is hard to describe but easy to identify. In a flanger, this swooping 'jet plane' sound moves up and down in pitch, but in a phaser it stays constant.
A phaser is very easy to program, and a little more difficult to construct physically. It consists of a very short (usually in the millisecond range) delay. The delayed signal is then added back on top of the original signal. The delay time is short enough so that at certain frequencies, the signal being added will be half a wavelength out of phase (hence the name), and will cancel out. At other frequencies, it will be close to one wavelength off, and will thus reinforce. Amplifying or attenuanting the delayed signal before mixing it back in will strengthen or weaken the effect.
Acoustically, this means a phaser will amplify certain pitches and attenuate others. This produces the characteristic 'hollow' and 'shrill' sound as certain pitches are dropped and other accentuated. The delay time will determine which pitches get affected. For example, a delay time of 5ms will amplify pitches near 200Hz, and cancel out ones at 100Hz. Also, even multiples of those pitches will be similarly affected.