A "time loop" or "temporal loop" is a plot device in which periods of time are repeated and re-experienced by the characters, and there is often some hope of breaking out of the cycle of repetition. Time loops are sometimes referred to as causal loops, but these two concepts are distinct. Although similar, causal loops are unchanging and self-originating, whereas time loops are constantly resetting. In a time loop when a certain condition is met, such as a death of a character or a clock reaching a certain time, the loop starts again, with one or more characters retaining the memories from the previous loop. Stories with time loops commonly center on the character learning from each successive loop through time. A popular term for a time loop is "groundhog day", a reference to the eponymous movie.