"Return Of The Repressed"

The term "Return of the repressed" is taken from Sigmund Freud's psychoanalytic theory.
In it's simplest terms the phrase means that traumatic events are often repressed but trauma generally strengthens the memories of the experience and then they are increased by emotional and physical triggers.

For example; If a person has had a traumatic experience happen to them in the past they will push that traumatic experience to the back of their mind and completely forget about it, but if they are then faced with seeing or experiencing something similar again in their lives, they are then reminded of the experience and are faced with the repressed memories. Often with experiences like this the victim can become depressed and even anti-social because of the repressed memories flooding back and causing them to face events of the past.

The "Return of the repressed" is a good basis for a story line in a horror movie because you often discover a character that has had a traumatic experience or have even been the person behind the traumatic experience this can often result in the killer coming back for the victim or even the victim becoming the killer.