Definition.
At its simplest, dissociation can be understood as the separation of one aspect of self from another aspect of self. This is a process which happens unconsciously, and is not something we do ‘on purpose.’ Though commonly associated with conditions like dissociative identity disorder (DID), dissociation is also associated with non-pathological phenomena.
A common example of dissociation arises when we are immersed in a good novel. People speak to us, but we don’t hear them. It’s raining outside, but we don’t notice the patter on the roof. We are, in essence, separated from our conventional self — the person who does the dishes, goes to work, or dances the tango. (And, in fact, your sultry tango-dancing self can also be said to be moderately dissociated from the ‘you’ that babysits your nephew, and the ‘you’ that works as a police officer.)
In the context of trauma, dissociation is a process by which we ‘shelve’ certain information differently than other information. For example, in traumatic dissociation, the part of self which holds the experiential memory of the traumatic event is separated by a dissociative boundary from the part of self which carries the emotions generated during the event. Dissociation can, in many cases, be understood as a ‘safety valve’ that keeps us from being overwhelmed by traumatic, undesirable, or overpowering experiences.