Reaction time is the amount of time that elapses between the presentation of a sensory stimulus and a behavioral response. Or, in other words, how fast can a person execute a mental operation needed by the current task.
For instance, a person driving a car sees an obstacle in the road and must react to it either by stopping or going around it. This is a simple task if sober, but under the influence of an intoxicating substance, it can be much more difficult. Therefore the reaction time of an intoxicated person is longer than the reaction time of a sober person. Likewise, younger people tend to have a faster reaction time than do older people.