Refers to any psychoacoustic effect as perceived by the listener. Think of these as optical illusions for the ears. Some engineering techniques take advantage of the fact that we do not always perceive audio accurately, or exactly as what is presented. For example, a commonly used psychoacoustic effect would be the "thickening" of a vocal track by "double-tracking" (q.v.), and then panning the two tracks hard left and hard right. The listener in this case would perceive the sound more fully in the "centre-stage" of the stereo spectrum.