Dictionary

Any activity designed to develop or hone a skill or ability.

Example: The teacher told us that the next exercise is to write an essay.

Activity intended to improve physical, or sometimes mental, strength and fitness.

Example: I do crosswords for mental exercise.

A setting in action or practicing; employment in the proper mode of activity; exertion; application; use.

Example: The law guarantees us the free exercise of our rights.

The performance of an office, ceremony, or duty.

Example: I assisted the ailing vicar in the exercise of his parish duties.

To exert for the sake of training or improvement; to practice in order to develop.

Example: to exercise troops or horses;  to exercise one's brain with a puzzle

To perform physical activity for health or training.

Example: I exercise at the gym every day.

To use (a right, an option, etc.); to put into practice.

Example: She is going to exercise her right to vote.

(now often in passive) To occupy the attention and effort of; to task; to tax, especially in a painful or vexatious manner; harass; to vex; to worry or make anxious.

Example: exercised with pain