Which term describes the maintenance of a steady internal state in the body?

In other words, the term homeostasis is being used to describe the body’s maintenance during resting conditions whereas the term steady state refers to the body’s maintenance during exercise; exercise is a stressor which disrupts homeostasis, also called homeostatic disturbance (e.g. an increase in temperature).

Homeostasis is the maintenance of a constant internal environment (the immediate surroundings of cells) in response to changes in:

the changing conditions of the external environment.
the changing conditions of the internal environment.

Homeostasis is a self adjusting mechanism involving feedback where the response to a stimulus alters the internal conditions and may itself become a new stimulus.

Homeostasis works to maintain the organism's internal environment within tolerance limits - the narrow range of conditions where cellular processes are able to function at a level consistent with the continuation of life.