The maximum weight you can move for exactly one clean repetition. The reference value behind almost every training plan.
Training plans work in percentages of your 1RM: strength adaptations kick in roughly above 85%, hypertrophy sits around 65–80%. You don't actually have to max out — a true one-rep attempt is draining and risky with imperfect form. Estimate it instead from a set of 3–8 clean reps (Epley: 1RM ≈ weight × (1 + reps / 30)). Two things people miss: the number moves with your daily form, sleep and stress, and it only applies to the exact lift you measured.