Variable Interval (VI) is a schedule of reinforcement in which reinforcement is provided after an average or variable amount of time has passed, rather than after a fixed duration. The exact amount of time between reinforcements varies, but it averages around a specific interval.
Example
In a classroom setting, a teacher might use a VI 5-minute schedule, where students receive praise for staying on task at variable intervals that average out to 5 minutes. Sometimes the reinforcement might occur after 3 minutes, sometimes after 7 minutes, but on average, it happens around every 5 minutes.