Is the principle whereby improvement occurs when increased demands are made upon the body.

plateau
progression
overload
specificity

Overload

Explanation:

-Plateau  is when the body adapts to the requirements of the workout a person is doing and needs to increase the demands to continue having results.

-Progression  means that people have to improve the demands of their workout to reach a level in which they are able to improve their fitness.

-Overload  refers to increasing the intensity of the workout to avoid adaptation and keep improving.

-Specificity means that the changes that occur in your body are related to the type of exercise you do.

According to this, the answer is that overload is the principle whereby improvement occurs when increased demands are made upon the body.