The greatest force a muscle or a group of muscles can produce in a single effort, usually expressed as a one-rep max.
It sits at one end of the force-velocity curve and underwrites everything else. A client whose squat max goes from 80 kg to 120 kg finds that their old 60 kg working sets became easy, because 60 kg went from 75 percent of max to 50.
Training it means loads above 85 percent for 1 to 5 reps, 3 to 5 minutes of rest, and total quality reps kept low, around 10 to 25 per session on the lift. Effort is high per rep and volume is deliberately not.
It responds to frequency and specificity. Squatting heavy twice a week beats once, and the max that improves is the max of the lift you practise, which is why a stronger leg press does not necessarily mean a stronger squat.