The unusually fast strength and size progress an untrained person makes in their first six to twelve months of training.
A beginner can add weight to the bar every session for months, because most of the early gain is neural: better coordination, better recruitment, less involuntary braking. A new lifter might squat 40 kg in week 1 and 90 kg by month 6, and roughly half of that is learning rather than tissue.
It ends gradually, not suddenly. Session-to-session progress becomes weekly, then monthly, and that is the signal to move from linear progression to double progression or a periodized block.
Two coaching implications. First, do not over-engineer a beginner program: almost anything works, so pick what teaches technique. Second, set the expectation early, because clients who are not warned that the rate slows down quit at month eight thinking they failed.