The act, fact, or process of exerting; active use of strength, power, etc.; exercise.
Energetic activity; effort.
The act or an instance of exerting, especially a strenuous effort.
Exertion is defined as physical or mental effort, or to the use of force or influence.
Doing an extremely hard crossword puzzle that taxes your brain is an example of mental exertion.
When you exercise your authority and forbid someone from going to a party, this is an example of an exertion of your authority.