Most people assume "performance enhancement" means taking steroids. This is simply not the case. Performance enhancement is more appropriately referred to as an ergogenic aid. According to the Exercise Physiology: Energy, Nutrition, & Human Performance textbook, ergogenic refers to the application of any physical, mechanical, nutritional, psychologic or pharmacologic procedure or aid that improves physical work capacity or athletic performance. So performance enhancement can be anything that improves your abilities for a specified activity. Examples of some ergogenic aids include: studying film of the opposing team, working on body position for cycling, tweaking the mechanics of your jump shot, wearing a squat suit for powerlifting competitions, visualizing yourself succeeding in a sport, etc. Something as simple as water itself could be thought of as an ergogenic aid because I can guarantee that without it, your performance in any situation would suffer. In most collegiate and professional sports however, the only performance enhancing aids that are regulated are in regards to nutritional supplements.
Katch, F. & Katch, V. & McArdle, W. (2007). Exercise Physiology: Energy, Nutrition, & Human Performance. 6th ed. USA: Lippincott Williams & Wilkins. 556.