Goniometer
A stereo meter that plots the left channel against the right as a shape, so width, balance and phase problems can be seen rather than calculated.
At heart it is an oscilloscope in XY mode: left drives one axis, right the other, and the picture is rotated 45 degrees so the mid signal runs vertically and the side signal horizontally. Mono shows up as a vertical line. Audio in the left channel only becomes a line leaning left. A wide stereo mix is a fuzzy cloud filling the display, and anything out of phase spreads sideways — exactly where you do not want it.
The name comes from angle measurement, and the tool is a broadcasting heirloom, from a world where a mono fold-down that cancelled half the programme was a genuine fault rather than an aesthetic choice. A goniometer almost always sits next to a correlation meter, which says the same thing with a single number.