The Collaborative International Dictionary of English v.0.48:

Idiot \Id"i*ot\ ([i^]d"[i^]*[o^]t), n. [F. idiot, L. idiota an
   uneducated, ignorant, ill-informed person, Gr. 'idiw`ths,
   also and orig., a private person, not holding public office,
   fr. 'i`dios proper, peculiar. See Idiom.]
   1. A man in private station, as distinguished from one
      holding a public office. [Obs.]
      [1913 Webster]

            St. Austin affirmed that the plain places of
            Scripture are sufficient to all laics, and all
            idiots or private persons.            --Jer. Taylor.
      [1913 Webster]

   2. An unlearned, ignorant, or simple person, as distinguished
      from the educated; an ignoramus. [Obs.]
      [1913 Webster]

            Christ was received of idiots, of the vulgar people,
            and of the simpler sort, while he was rejected,
            despised, and persecuted even to death by the high
            priests, lawyers, scribes, doctors, and rabbis. --C.
                                                  Blount.
      [1913 Webster]

   3. A human being destitute of the ordinary intellectual
      powers, whether congenital, developmental, or accidental;
      commonly, a person without understanding from birth; a
      natural fool. In a former classification of mentally
      retarded people, idiot designated a person whose adult
      level of intelligence was equivalent to that of a
      three-year old or younger; this corresponded with an I.Q.
      level of approximately 25 or less.
      [1913 Webster +PJC]

            Life . . . is a tale
            Told by an idiot, full of sound and fury,
            Signifying nothing.                   --Shak.
      [1913 Webster]

   4. A fool; a simpleton; -- a term of reproach.
      [1913 Webster]

            Weenest thou make an idiot of our dame? --Chaucer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

idiot
    n 1: a person of subnormal intelligence [syn: idiot,
         imbecile, cretin, moron, changeling, half-wit,
         retard]

The Devil's Dictionary (1881-1906):

IDIOT, n.  A member of a large and powerful tribe whose influence in
human affairs has always been dominant and controlling.  The Idiot's
activity is not confined to any special field of thought or action,
but "pervades and regulates the whole."  He has the last word in
everything; his decision is unappealable.  He sets the fashions and
opinion of taste, dictates the limitations of speech and circumscribes
conduct with a dead-line.