Phase correlation meter
A meter showing how similar the left and right channels are, on a scale from +1 to −1 – and therefore whether the mix survives being played in mono.
At +1 the channels are identical, which is mono. Around zero they are unrelated, normal for wide stereo recordings. Below zero they start working against each other, and once the signal is summed to mono – a phone speaker, a club's sub system, an old radio – content cancels out. A bass with flipped polarity in one channel can vanish entirely.
The meter almost always sits next to a goniometer or vectorscope, which draws the same information as a shape: a vertical line for mono, a horizontal one for out-of-phase. It is an old broadcast tool that found new purpose once stereo-widening plug-ins made it easy to build a mix that simply isn't there in mono.