The specific part of a lift where the bar slows down or stops, caused by the joint angles there being the weakest position in the range.
It is a position, not a muscle. Most benchers stick a few inches off the chest, most squatters just above parallel, most deadlifters either at the floor or at the knee. The bar moves fine before and after; it stalls there.
Diagnose it by watching, then train the position. A bench that stalls off the chest wants paused bench presses, two seconds on the chest, at about 80 percent of the normal working weight. A deadlift that stalls at the knee wants block pulls or a pause below the knee. A squat that stalls out of the hole wants pause squats.
Give it four to six weeks in a block and then test the full lift. Sticking points move as strength changes, so the fix is temporary by design.