Source : Webster's Revised Unabridged Dictionary (1913)

Psychosis \Psy*cho"sis\, n. [NL. See {Psycho-}.]
   1. Any vital action or activity. --Mivart.

   2. (Med.) A disease of the mind; especially, a functional
      mental disorder, that is, one unattended with evident
      organic changes.

Source : WordNet®

psychosis
     n : any severe mental disorder in which contact with reality is
         lost or highly distorted
     [also: {psychoses} (pl)]