The phase of a rep where the muscle shortens under load, such as standing up out of a squat or curling a dumbbell upward.
It is the half of the rep that fails first. A client who cannot finish a set stalls on the way up, never on the way down, which is why they can still lower a weight they cannot lift.
Concentric force is lower than eccentric force by roughly 20 to 50 percent. That gap is the basis of forced reps, eccentric overload and drop sets: a lifter who fails a 100 kg bench concentrically can still control 110 kg on the way down.
Speed matters here more than on the eccentric. Intending to move the bar fast on the concentric, even when the weight is heavy enough that it moves slowly, recruits more high-threshold motor units than lifting it deliberately slowly.