The number of repetitions of an exercise that can be completed with a given weight. Such as 1RM, for 1 Repetition Maximum, means the lifter can only perform one repetition before reaching momentary concentric muscle failure in that specific exercise with that particular weight. In other words, not another single repetition can be performed.
RM is also referred to as Voluntary Maximal Muscle Action. This type of action does not refer to the movement of the muscle but to the intensity of the resistance. When a muscle reaches VMMA, it is moving against as much weight as the current fatigue level will allow.