A neuropsychiatric disorder characterized by one or more of the following essential features: immobility, mutism, negativism (active or passive refusal to follow commands), mannerisms, stereotypies, posturing, grimacing, excitement, echolalia, echopraxia, muscular rigidity, and stupor; sometimes punctuated by sudden violent outbursts, panic, or hallucinations. This condition may be associated with psychiatric illnesses (e.g., schizophrenia; mood disorders) or organic disorders (neuroleptic malignant syndrome; encephalitis, etc.). (From DSM-IV, 4th ed, 1994; APA, Thesaurus of Psychological Index Terms, 1994)