Phase

In physics and mathematics, the phase of a periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is denoted ϕ(t) and expressed in such a scale that it varies by one full turn as the variable t goes through each period (and F(t) goes through each complete cycle). It may be measured in any angular unit such as degrees or radians, thus increasing by 360° or 2π as the variable t completes a full period.

This convention is especially appropriate for a sinusoidal function, since its value at any argument t then can be expressed as the sine of the phase ϕ(t), multiplied by some factor (the amplitude of the sinoid). (The cosine may be used instead of sine, depending on where one considers each period to start.)