A scale for perceived effort. In strength training it usually runs 1–10, where RPE 8 means you had two more reps in the tank.
RPE fixes the core flaw of rigid percentage plans: they prescribe 80% whether you slept six hours or just came back from holiday. With RPE you steer by daily readiness — same target effort, adjusted load. Two scales are in common use, don't mix them up: the 1–10 Borg CR10 version (strength) and the classic Borg 6–20 scale (endurance, historically tied to heart rate). Beginners almost always underrate their RPE — what feels like a 9 early on is often a 7.