The force a muscle fiber experiences while contracting under load, currently considered the main driver of muscle growth.
Tension is sensed inside the fiber and triggers the signalling that leads to protein synthesis. High tension comes either from a heavy load or from a light load lifted when the muscle is already fatigued, which is why sets of 5 at 85 percent and sets of 20 taken near failure both build muscle.
The practical consequence: proximity to failure is what makes a set count, not the weight on the bar. Three sets of 12 at RPE 9 and three sets of 6 at RPE 9 do similar work; three sets of 12 at RPE 5 do not.
Range matters too, since tension is highest when the muscle is lengthened. Full-range reps and lengthened partials both exploit that.