RPE
RPE = Rate of Perceived Effort. It is basically a number that lets you know how hard the set you just did was:
RPE @ 7 - very explosive, and at least 3 reps in reserve, this can be used as a speed-training and/or deloading
RPE @ 8 - heavy but still quite explosive, about 2 reps in reserve
RPE @ 9 - now it's very heavy, and if watched as a replay, you are slower than RPE8. You can clearly go one more rep, but not for two
RPE @ 10 - your absolute maximum effort, you are pretty much at your 1RM range. If you need help completing a rep, you are definitely in RPE10
There is an inherent assumption with the above regarding form being done well. An RPE9 squat that is clean and nice whereas an RPE9 squat with your hips shoot up and you good morning the weight up are not the same. It is easy to think you pulled off a RPE 8 when really it was an 8.5 or a 9.
Bron: http://examine.com/autoregulation.php