Reaction time is a measure of how long it takes someone or something to respond to a stimulus, make a decision, or perform a task. It is frequently used in psychology experiments. For example, a researcher interested in mental rotation can measure how long it takes subjects to decide whether a displayed image that has been rotated various amounts matches one shown before.

In high school we use to have one person hold a yard stick and the other with his hands outstreched and around the yard stick. The person holding the yard stick would let go, and the other person would need to clap his hands shut, before it hit the ground. You could see how many inches it fell before you stopped it, and then judge that based on others measurments, to see who had the quickest reaction time.

In drag racing, the amount of time between the appearance of the last amber light and the moment when the driver first crosses the start line. A pro tree has a "perfect reaction time of 0.4 seconds, meaning that the time between the amber light and the green light is 0.4 seconds (if the driver leaves before the green light, he red lights and loses the race). A handicap tree has a perfect reaction time of 0.5 seconds. An experienced drag racer will usually have a reaction time no more than 0.67 or so.