Final answer:
The overload principle is the fitness principle characterized by stressing a body system so that it will adapt.
Step-by-step explanation:
The principle characterized by stressing a body system so that it will adapt is the overload principle. The overload principle states that in order to improve fitness and increase strength or endurance, you must gradually and systematically increase the amount of stress you place on your body during exercise. By doing so, the body will adapt and become stronger and more efficient in that specific area.