Definition
Fatigue that builds across sessions faster than recovery clears it, so performance drops even though each session felt fine.
The fitness-fatigue model explains it: every session adds both fitness and fatigue, fatigue decays faster than fitness, and performance is what is left after fatigue clears. String hard sessions close enough together and the fatigue never clears.
The signs are quiet: RPE creeping up at the same load, sleep and mood a little worse, small aches that move around. It is different from overtraining, which is rare and needs weeks off.
Manage it by planning deloads, not stacking failure sets, and watching total weekly volume rather than any one session.