Reaction time was the amount of time it took for an individual's reflexes to react to stimuli.
In 2266, following one simulated attack, Dave Bailey informed all decks of USS Enterprise that they should ready themselves to improve their reaction time on the second attack. (TOS: "The Corbomite Maneuver")
While training for entrance into Starfleet Academy's Academy preparatory program in 2371, Nog managed to improve his reaction time by nearly twenty percent. According to him, he had five percent more before he reached the cutoff. (DS9: "Facets")