"Return of the Repressed"

Definition: the process whereby psychological elements, preserved in the unconscious part of your mind, tend to reappear or become aware by the conscious part of your mind.

Relation to the Horror genre: The return of the repressed is seen as a sub-genre of horror. As a theme, it is the idea of the return of something we had thought we had left behind or repressed or a thing of the past re-emerging to the present. This would be mostly used in psychological thrillers like ghost or demonic horror movies. I believe it was used as a theme in Dr Jekyll and Mr Hyde as Dr Jekyll's inner evil emerged.