Phase describes where in its cycle a periodic waveform is at any given time. The relationship in time of two or more waveforms with the same or harmonically related periods gives us a measurement of their phase difference. Phase cancellation occurs when two signals of the same frequency are out of phase with each other resulting in a net reduction in the overall level of the combined signal. If two identical signals are 100% or 180 degrees out of phase they will completely cancel one another if combined. When similar complex signals (such as the left and right channel of a stereo music program) are combined phase cancellation will cause some frequencies to be cut, while others may end up being boosted.
Phase and phase difference is a real-world issue in areas such as electrical wiring of audio equipment, signal path, and microphone placement during the recording process. Phase reversal can be a serious compromise of sound quality or a special effect affecting the perceived spaciousness of the sound depending on the context of its occurrence.