Definition.
Dissociation is a process by which we ‘shelve’ certain, often traumatic, information differently than other information. In dissociation, memory may be ‘held’ separately from the feelings experienced at the time the memory was formed. Dissociation can, in many cases, be understood as a ‘safety valve’ that keeps us from being overwhelmed by traumatic or strongly undesirable experiences.
An expanded reading about dissociation can be found in the article, Multiple personality: The complex psychology of dissociative identity disorder.