The Collaborative International Dictionary of English v.0.48:

Illuminati \Il*lu`mi*na"ti\, n. pl. [L. illuminatus. See
   Illuminate, v. t., and cf. Illuminee.]
   Literally, those who are enlightened; -- variously applied as
   follows:
   [1913 Webster]

   1. (Eccl.) Persons in the early church who had received
      baptism; in which ceremony a lighted taper was given them,
      as a symbol of the spiritual illumination they has
      received by that sacrament.
      [1913 Webster]

   2. (Eccl. Hist.) Members of a sect which sprung up in Spain
      about the year 1575. Their principal doctrine was, that,
      by means of prayer, they had attained to so perfect a
      state as to have no need of ordinances, sacraments, good
      works, etc.; -- called also Alumbrados,
      Perfectibilists, etc.
      [1913 Webster]

   3. (Mod. Hist.) Members of certain associations in Modern
      Europe, who combined to promote social reforms, by which
      they expected to raise men and society to perfection, esp.
      of one originated in 1776 by Adam Weishaupt, professor of
      canon law at Ingolstadt, which spread rapidly for a time,
      but ceased after a few years.
      [1913 Webster]

   4. Also applied to:
      (a) An obscure sect of French Familists;
      (b) The Hesychasts, Mystics, and Quietists;
      (c) The Rosicrucians.
          [1913 Webster]

   5. Any persons who profess special spiritual or intellectual
      enlightenment.
      [1913 Webster]

The Devil's Dictionary (1881-1906):

ILLUMINATI, n.  A sect of Spanish heretics of the latter part of the
sixteenth century; so called because they were light weights --
_cunctationes illuminati_.