The Collaborative International Dictionary of English v.0.48:

paranoia \par`a*noi"a\ (p[a^]r`[.a]*noi"[.a]), n. [NL., fr. Gr.
   para`noia.]
   1. (Med.) A chronic form of insanity characterized by very
      gradual impairment of the intellect, systematized
      delusion, and usually by delusions of persecution or
      mandatory delusions producing homicidal tendency. In its
      mild form paranoia may consist in the well-marked
      crotchetiness exhibited in persons commonly called
      "cranks." Paranoiacs usually show evidences of bodily and
      nervous degeneration, and many have hallucinations, esp.
      of sight and hearing.
      [Webster 1913 Suppl.]

   2. Unwarranted suspicion of the motives of others; -- used
      colloquially in a non-technical sense. [Colloq.]
      [PJC]

WordNet (r) 3.0 (2006):

paranoia
    n 1: a psychological disorder characterized by delusions of
         persecution or grandeur

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "paranoia":
   alternating personality, brain disease, catatonia,
   catatonic schizophrenia, crack-up, dementia praecox,
   depersonalization, disconnection, dissociation,
   dissociation of personality, double personality, dual personality,
   emotional disorder, emotional dissociation, emotional instability,
   functional nervous disorder, hebephrenia,
   hebephrenic schizophrenia, insanity, maladjustment,
   manic-depressive psychosis, melancholia, mental disorder,
   mental dissociation, mental illness, multiple personality,
   nervous breakdown, nervous disorder, neurosis,
   paranoid personality, paranoid psychosis, paraphrenia,
   personality disorder, personality disorganization,
   problems in living, psychosis, reaction, schizoid personality,
   schizoidism, schizophrenia, schizothymia, social maladjustment,
   split personality