1. A practice or exercise to test or improve one's fitness for athletic competition, ability, or performance
2. A test of one's ability, capacity, stamina, or suitability
3. A series of exercises done in order to improve your health or physical appearance is a workout. Workout clothes are clothes you wear while exercising.
4. When you say that you gave a mechanical device or system a workout, you mean that you caused it to perform some lengthy or difficult work.