Reaction time refers to the amount of time it takes for an individual to respond or react to a stimulus, such as a sound, sight, or touch. It is measured from the moment the stimulus occurs until the response begins and can vary depending on factors such as age, experience, stress levels, and physical conditioning. In general, reaction times are quickest in young adults and tend to slow down with age due to declines in cognitive function and motor skills. Reaction time is an important measure of performance in various fields, including sports, medicine, and psychology, where it can provide insights into an individual's ability to process information and react quickly under pressure.