The Collaborative International Dictionary of English v.0.48:

Illustrious \Il*lus"tri*ous\, a. [L. illustris, prob. for
   illuxtris; fr. il- in + the root of lucidus bright: cf. F.
   illustre. See Lucid.]
   1. Possessing luster or brightness; brilliant; luminous;
      splendid.
      [1913 Webster]

            Quench the light; thine eyes are guides illustrious.
                                                  --Beau. & Fl.
      [1913 Webster]

   2. Characterized by greatness, nobleness, etc.; eminent;
      conspicuous; distinguished.
      [1913 Webster]

            Illustrious earls, renowened everywhere. --Drayton.
      [1913 Webster]

   3. Conferring luster or honor; renowned; as, illustrious
      deeds or titles.

   Syn: Distinguished; famous; remarkable; brilliant;
        conspicuous; noted; celebrated; signal; renowened;
        eminent; exalted; noble; glorious. See Distinguished,
        Famous.
        [1913 Webster]

WordNet (r) 3.0 (2006):

illustrious
    adj 1: widely known and esteemed; "a famous actor"; "a
           celebrated musician"; "a famed scientist"; "an
           illustrious judge"; "a notable historian"; "a renowned
           painter" [syn: celebrated, famed, far-famed,
           famous, illustrious, notable, noted, renowned]
    2: having or conferring glory; "an illustrious achievement"

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "illustrious":
   acclaimed, bright, brilliant, celebrated, charismatic, conspicuous,
   distinguished, eminent, esteemed, famed, famous, glamorous,
   glorious, great, honored, important, lofty, lustrous, magic,
   notable, noted, numinous, outstanding, prominent, radiant,
   renowned, respected, resplendent, shining, signal, splendent,
   splendid, splendorous, splendrous, striking, sublime, venerable,
   well-known



The Devil's Dictionary (1881-1906):

ILLUSTRIOUS, adj.  Suitably placed for the shafts of malice, envy and
detraction.