1RM
One-Rep Max
The heaviest weight you can lift once with good technique.
What it means in a class
Your 1RM is the maximum load you can move for a single rep on a given lift. Programming often uses percentages of it — 'build to 75% of your 1RM' — so knowing roughly where yours sits is useful.
You'll also see 3RM and 5RM, meaning the heaviest you can lift for three or five reps. These are often safer and just as informative for newer lifters.
On the whiteboard
"Work up to a heavy single, roughly 90% of your 1RM."