Phase

Phase is the measure of progression of a periodic wave. Phase identifies the position at any instant which a periodic wave occupies in its cycle. It can also be discribed as the time relationship between two signals.

and

A portion of a material system whose properties and composition are homogeneous and which is physically distinct from other parts of the system.

See also: Frequency, Group Delay, In Phase, Lag, Phase Angle, Phase Change, Phase Diagram, Wavelength.