Answer:
A within-subject variable is a type of variable in which the same subjects are used to measure multiple levels or conditions of the variable. One example of a within-subject variable would be measuring a person's reaction time to a stimulus both before and after a training intervention.
For example: measuring a person's reaction time to a visual stimulus before and after a training program in which the participant learns to react faster to the stimulus, here reaction time is the within-subject variable.