The fractional part (portion) of a period through which the time variable of a periodic quantity (alternating electric current, vibration) has moved, as measured at any time instant from an arbitrary time origin. In the case of a sinusoidally varying quantity, the time origin is usually assumed to be the last point at which the value of the quantity was zero, passing from a negative to a positive value. It is customary to choose the origin so that the fractional part of the period is less than unity.