Periodization is structured variation in training load over time. The body adapts to stress, then requires new stress to adapt further — but it also requires recovery.
Basic structure: mesocycle (3–6 week block of progressive loading) → deload week (reduced volume, maintained intensity) → new mesocycle at higher baseline.
Hans Selye's General Adaptation Syndrome provides the biological basis: stress → alarm → resistance (adaptation) → exhaustion. Periodization manages the cycle to stay in the resistance phase without reaching exhaustion.