While we are used to thinking of electric voltage and current as being constant (for instance, receiving 120 V service) in reality the voltage and current of an electric line are continually fluctuating from positive to negative values. Phase in a power system describes a full cycle in voltage or current as shown on the following diagram:
Phase difference refers to the difference between where two different curves cross zero. Often a single-phase is described as being 360 degrees. In this case the difference in phase is referred to as the phase angle.
In power systems the phase angle is often used to describe how much the voltage curve leads the corresponding current curve. Instruments such as phasor measurement units (PMUs) are used by system operators to monitor the phase angle at various locations, which gives an indication of whether a power system is functioning as desired.