A phasor describes the phase of a particle in a simple harmonic motion or a wave motion. It is a vector in the plane, that rotates with a constant angular velocity around the origin. The constant length of the vector gives the amplitude and the angle it makes with the x-axis gives the phase angle. The displacement is given by the y-value.

The additional information, such as the velocity and the acceleration of the particle at any given time, may be read from this picture. For many purposes, a more convenient description of the wave motion is provided by complex numbers. The algebra is greatly facilitated by the complex exponential function.

This is a simple example of a phase diagram.