What exactly is depersonalisation?

Depersonalization: Depersonalization is an alteration in the perception or experience of the self, so that one feels detached from -- or as an outside observer to -- one's mental processes or body. It's as if you're in a dream, or feeling unreal. It can range from sporadic and fleeting experiences in youth -- which are quite common -- all the way to the severe and disabling pathological depersonalization disorder.