The state in which the body's systems are functioning smoothly and in equilibrium

The state in which the body's systems are functioning smoothly and in equilibrium is called homeostasis.  

Homeostasis is the tendency to maintain a balanced (relatively constant) internal environment and it can also be defined as the stable state of an organism. Homeostasis is maintained at many levels, not just the level of the whole body (blood sugar levels, pH, fluid balance…). Each of these is controlled by one or more homeostatic mechanisms (feedbacks for example).