Polarity inversion
Flipping a signal upside down — every sample multiplied by −1 — usually so two microphones stop cancelling each other.
The button is nearly always labelled Ø and called "phase", which is wrong in a way engineers love to point out. Phase is a time shift and depends on frequency; a polarity flip affects all frequencies at once and adds no delay at all. It won't cure a comb filter, but it does cure the classic case: one mic above the snare and one below. When the head moves down it pushes on one and pulls on the other, and the sum sounds thin until one of them is flipped.
The fault can also be in the wiring. Balanced XLR cables carry hot and cold conductors, and until the AES settled on pin 2 as hot in the early 1990s, manufacturers wired them differently. Fold a mix down to mono and anything flipped the wrong way gives itself away quickly.