Dissociative identity disorder
Dissociation is a symptom.  Most people experience normal dissociation, where they stop paying attention to what is happening around them.  People with DID have pathological dissociation, which is much more serious.  It involves problems with memory and attention that make it hard for people to work, shop and have relationships with other people.  Everyone can dissociate but some people can do it much more easily and to an extreme that can be unhealthy.  Some doctors think trauma causes dissociation which is seen in disorders like DID.