Dissociation is essentially a lack of integration of psychological processes. In other words, a person who is dissociated has two or more things going on in his mind that are disconnected.
For example, a person who is dissociated may be unaware that he is eating or that he is staring at someone. It can also involve lack of awareness of location: The person may not be conscious of being in a certain room or building.
In psychotherapy, it is common for a person to have little or no emotional reaction to memories of painful events. This is dissociation -- a disconnection between the memories and the attached emotions.
In many cases, the dissociated person's awareness focuses on something not present in the moment. It could be another person or a fantasy about what might be happening elsewhere. Common examples of dissociation include not remembering the last several miles while driving, or reading the same paragraph several times without understanding it.
In particular, those with bulimia nervosa or binge-eating disorder seem to experience dissociation while binging; there is a lack of awareness of the amount of food consumed. The same occurs on a lesser scale when eating while watching television.
Sources:
Engelberg MJ, Steiger H, Gauvin L, and Wonderlich SA. Binge antecedents in bulimic syndromes:
An examination of dissociation and negative affect. International Journal of Eating Disorders 40 (2007): 531-536.
Farrington A, Waller G, and Neiderman M. Dissociation in adolescent girls with anorexia: Relationship to comorbid psychopathology. Journal of Nervous and Mental Disease 190 (2002): 746-751.
