Homeostasis

Homeostasis is the set of regulatory processes that hold internal variables — temperature, blood glucose, pH, fluid balance, blood pressure — within narrow ranges. It works through negative feedback: a deviation is detected, a correction applied, the variable returns towards its set point.

It matters because training works by disturbing it. Exercise pushes the body away from its comfortable resting state, and adaptation is the body making itself better able to cope with that disturbance next time. The stimulus is the disruption; the fitness is the response.

In practice this also explains why adaptation stalls. Once a given disturbance no longer threatens homeostasis, there is nothing to adapt to — which is the physiological argument behind progressive overload.