What is the definition of a performance-enhancing substance?

A performance-enhancing substance is defined as any substance that is taken specifically to increase athletic ability, which aligns perfectly with the correct answer. These substances can range from legal supplements to illegal drugs and are often used by athletes to improve strength, stamina, speed, or overall athletic performance.

In the context of this question, the focus is on the goal of enhancing performance specifically in sports or physical activities. While options related to aesthetic appeal, nutritional supplements, and hydration are important components of an athlete's regimen, they do not specifically target the enhancement of athletic performance itself. Therefore, the designation of a substance as performance-enhancing is primarily connected to its ability to improve capabilities directly related to athletic performance. This distinction is crucial for understanding the nature of such substances and their implications in competitive sports.