Traditionally, a device used to measure angles. There have been goniometers designed for many purposes throughout history, such as communications, medicine, and many more. In audio, a goniometer is a display that shows the relationship between the phase of two or more signals. The signals are plotted as a two-dimensional graph on two axes, which shows the correlation between them. The “shape” of the display indicates phase coherency. A vertical line indicated 100% in-phase signals. The farther the display moves from a vertical line, the farther the signals are out of phase.
Using a goniometer — which is a display found both in the analog domain and increasingly in software plug-ins — the relative levels, stereo/surround relationship, and mono compatibility of signals can be determined.