This is the current definition of psychosis (retrieved June 2014) from the Oxford English Dictionary (the OED), the standard reference work of the English language:
1. Med. and Psychol. Originally: any kind of disordered mental state or mental illness. Later: spec. severe mental illness, characterized by loss of contact with reality (in the form of delusions and hallucinations) and deterioration of intellectual and social functioning, occurring as a primary disorder or secondary to other diseases, drug ingestion, etc.; an instance of this. Cf. neurosis n. 1.