1RM (one-rep max)
Your one-rep max is the heaviest weight you could lift once with good form. Estimating it from a set of several reps avoids the risk of a true maximal attempt, at the cost of accuracy that falls as rep count rises.
Example
100 kg for 5 reps estimates to 116.7 kg by Epley and 112.5 kg by Brzycki. Above about ten reps both equations become unreliable.