Someone recently asked me this within their trauma diary, at which point also struck me, that if one did not know, then others also would not completely know, or possibly understand what dissociation was, so here it is.

Dissociation is not something you do voluntarily; instead involuntarily. You cannot decide or plan to dissociate. To put it simply, "tuning out" "zoning out" "spacing out" are colloquial names for dissociation. Dissociation refers to the ways in which people disconnect from traumatic experiences emotionally, mentally or both. There are generally four types of dissociation, being:

1. Senses and emotions disconnect - you may see something, but you don't react, or you don't react as might be expected. For example, children of battered women might dissociate in this manner, ie. whilst their mother is being beaten, they watch TV, eat dinner, or something as though nothing is going on.
2. Depersonalization - where you feel like a robot or thing, not a person. For example, you could be severely injured, but you can't feel much pain, because you feel more robotic than human. A thing, rather than a person.
3. Detachment - basically where you feel your floating, or distant from an emotion or event. Example, this is usual where you see, hear, smell and comprehend a traumatic event around you, but only in part, missing pieces because your detaching from the environment.
4. Compartmentalization - generally found in those with multiple personalities, where the memories are stored or compartmentalized in different personalities, so one personality remembers an event, whilst another knows nothing about that event, but about other events, vice versa.