Glossary definition of 'Phase'(waves) For sinusoidal waveforms, the phase is the fraction of a complete cycle since a specified point in time. Phase is usually measured in degrees or radians since a complete cycle of a sinusoidal function takes place over 360 degrees or 2π radians.
The phase of a signal is only of significance when it is expressed relative to some reference. For example, inter-channel phase is the difference in phase between two signals of the same frequency on two separate channels. A phase shift occurs as a result of a delay in a signal, in which case we are interested in the change of phase from the original signal.
Phase is perhaps easier to understand with rotating motion such as the rotation of the earth. The earth is divided into 360 degrees longitude and rotates round its own axis once in 24 hours, or at a rate of 15° per hour. All points on the earth are rotating with the same "frequency" but at any given moment, the different places on the earth are in a different "phase" of their day - some are in darkness, some are in light. New York is 71° "behind" London - the sun will pass overhead 4.733 hours after it passes over London. This difference in phase of the days gives rise to time zones.