Dictionary definition
From the Oxford English Dictionary.
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.