In its narrowest sense, "mentally disordered offender" refers to the approximately twenty thousand persons per year in the United States who are institutionalized as not guilty by reason of insanity, incompetent to stand trial, and mentally disordered sex offenders, as well as those prisoners transferred to mental hospitals.

Citation
John T. Monahan & Henry J. Steadman, eds., Mentally Disordered Offenders: Perspectives from Law and Social Science, Plenum Press (1983).