Phase
The Phase control on a mixer channel inverts the waveform. Use it when two copies of the same sound get thinner as soon as both play.
Phase is the position of a waveform in its cycle. Two copies of the same sound, one slightly delayed, can cancel: a peak in one lands in a trough in the other, and the level drops. You usually hear that as a loss of bass, or as a hole that moves when you pan.
The Phase control on a mixer channel inverts the signal. Every peak becomes a trough. Gain stays where it was. If two microphones, or a sound and a copy of it, get thin together, invert one of them. If the low end comes back, they were cancelling.
Heisenberg uses the same word for something else. Phase modulation there moves one operator’s phase continuously to make new harmonics. That path is documented on Heisenberg. The mixer button is a single flip of polarity.