Depersonalization means altering of perception that causes people to temporarily lose a sense of their own reality; most prevalent in people with the Dissociative disorders . There is usually a feeling of being outside observers of their own behavior.
Other /More definition:
Depersonalization is defined as an altered experience of the self, ranging from feeling that one's body is not connected to one's mind to the feeling that one is not real. It is also defined as the altering of perception that causes people to temporarily lose a sense of their own reality; most prevalent in people with the dissociative disorders. There is usually a feeling of being outside observers of their own behavior.