Intermittent Reinforcement is a conditioning schedule in which a reward or punishment (reinforcement) is not administered every time the desired response is performed. This differs from continuous reinforcement which is when the organism receives the reinforcement every time the desired response is performed. For example, on a continuous reinforcement schedule a mouse who pulls a lever would receive food (reinforcement) every single time it pulled the lever. On an intermittent reinforcement schedule the mouse would only receive food every few times (it is typically random and unpredictable). There is an increased likelihood the desired behavior will continue with intermittent reinforcement conditioning and the behavior lasts longer than continuous reinforcement.
Intropsych.com is useful to elucidate why behavior persists with intermittent reinforcement:
This explains so much.
Some days, we are merely laboratory animals, trying and trying in the hope we will receive again the things we crave, even though we are not guaranteed any consistency or success.
(I cannot expound further; some things are best left unsaid. Yet even to understand is helpful.)
I joke on occasion that I was a rodent in a former life. Sometimes, though, I suspect there may be more than a grain of truth about it.