Derealization (sometimes abbreviated as DR) is an alteration in the perception or experience of the external world so that it seems unreal. Other symptoms include feeling as though one’s environment is lacking in spontaneity, emotional coloring and depth. It is a dissociative symptom of many conditions, such as psychiatric and neurological disorders, and not a standalone disorder. It is also a transient side effect of acute drug intoxication, sleep deprivation, and stress.
Derealization is a subjective experience of unreality of the outside world, while depersonalization is unreality in one’s sense of self. Although most authors currently regard derealization (surroundings) and depersonalization (self) as independent constructs, many do not want to separate derealization from depersonalization. The main reason for this is nosological, because these symptoms often co-occur, but there is another, more philosophical reason: the idea that the phenomenological experience of self, others, and world is one continuous whole. Thus, feelings of unreality may blend in and the person may puzzle over deciding whether it is the self or the world that feels unreal to them.
Read more on Wikipedia