The cyclic relationship between two waveforms of the same fundamental frequency. The point at which the waveform crosses the X-axis on the upward slope is usually designated as the zero point for measuring phase relationships. Phase differences are most commonly expressed as degrees of a circle. So, for instance, if there are two waveforms of the same frequency, and one is a quarter-cycle ahead of the other, it is said to be 90 degrees ahead (90 degrees being one-quarter of a circle).
Two signals of the same fundemental frequency, but with a non-zero phase relationship, will experience reinforcement of some overtone frequencies and cancellation of others, altering the timbre of the mixed signals. If the phase relationship changes, the set of frequencies which are reinforced and cancelled changes. This is most easily illustrated with sine waves; two sines with a phase difference of less than 90 degrees will reinforce each other to produce a sine wave with a lower peak amplitude. As the phase relationship increases to 180 degrees, the two sines will cancel each other out resulting in no signal. Anything above 270 degrees will again produce a lower peak amplitude, and finally the two waveforms return to 0 degrees to increase each other's amplitude again.