A(n) schedule is a timetable in which a behavior is reinforced after a variable amount of time has elapsed.

variable-interval schedule

Explanation:

A variable-interval schedule is one of the schedules of partial reinforcement in which behavior or a response is rewarded after a variable amount of time, which is unpredictable, has elapsed. The time interval for rewarding behaviors continuously, after they have been initially established, is varied. For example, rewarding a lab rat for pressing a bar at first after one minute elapsed, then varying the time interval to 4 minutes for the same response to be rewarded and so on. The time interval is varied. Variable-interval schedule produces a slow but steady rate of response that is resistant enough to extinction.