Reaction time is the lapse of time between muscular movement and external stimuli.

Reaction time is indeed defined as the interval between the onset of a stimulus and the initiation of a response, typically a muscular movement. It reflects an individual's ability to respond promptly to an external stimulus, which is crucial in many aspects of physical performance, such as in various sports where quick decision making and prompt execution of movements can greatly influence outcomes. A shorter reaction time indicates a more efficient nervous system and improved coordination, while a longer reaction time may suggest delays in processing or response. Understanding this concept is important for improving athletic performance and skill-related fitness.