is an unfounded or exaggerated distrust of others, sometimes reaching delusional proportions. Paranoid individuals constantly suspect the motives of those around them, and believe that certain individuals, or people in general, are "out to get them."
Paranoid perceptions and behaviour may appear as features of a number of mental illnesses, including depression and dementia, but are most prominent in three types of psychological disorders: paranoid , delusional disorder (persecutory type), and paranoid personality disorder (PPD).
Individuals with paranoid schizophrenia and persecutory delusional disorder experience what is known as persecutory delusions: an irrational, yet unshakable, belief that someone is plotting against them. Persecutory delusions in paranoid schizophrenia are bizarre, sometimes grandiose, and often accompanied by auditory hallucinations. Delusions experienced by individuals with delusional disorder are more plausible than those experienced by paranoid schizophrenics; not bizarre, though still unjustified. Individuals with delusional disorder may seem offbeat or quirky rather than mentally ill, and, as such, may never seek treatment.
Persons with paranoid tend to be self-centered, self-important, defensive, and emotionally distant. Their paranoia manifests itself in constant suspicions rather than full-blown delusions. The disorder often impedes social and and career advancement. Some individuals with PPD are described as "litigious," as they are constantly initiating frivolous law suits. PPD is more common in men than in women, and typically begins in early adulthood.
A definition of Paranoia from the World of Health - http://www.bookrags.com/Paranoia