Quick definitions from WordNet (depersonalisation)
▸ noun: representing a human being as a physical thing deprived of personal qualities or individuality
▸ noun: (existentialism) a loss of personal identity; a feeling of being an anonymous cog in a stupid social machine
▸ noun: emotional dissociative disorder in which there is loss of contact with your own personal reality accompanied by feelings of unreality and strangeness
▸ noun: representing a human being as a physical thing deprived of personal qualities or individuality
▸ noun: (existentialism) a loss of personal identity; a feeling of being an anonymous cog in a stupid social machine
▸ noun: emotional dissociative disorder in which there is loss of contact with your own personal reality accompanied by feelings of unreality and strangeness