The Collaborative International Dictionary of English v.0.48:

Immaculate \Im*mac"u*late\, a. [L. immaculatus; pref. im- not +
   maculatus, p. p. of maculare to spot, stane, fr. macula spot.
   See Mail armor.]
   Without stain or blemish; spotless; undefiled; clear; pure.
   [1913 Webster]

         Were but my soul as pure
         From other guilt as that, Heaven did not hold
         One more immaculate.                     --Denham.
   [1913 Webster]

         Thou sheer, immaculate and silver fountain. --Shak.
   [1913 Webster]

   Immaculate conception (R. C. Ch.), the doctrine that the
      Virgin Mary was conceived without original sin. --
      Im*mac"u*late*ly, adv. -- Im*mac"u*late*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Immaculate Conception
    n 1: Roman Catholic holy day first celebrated in 1854 [syn:
         Immaculate Conception, December 8]
    2: (Christianity) the Roman Catholic dogma that God preserved
       the Virgin Mary from any stain of original sin from the
       moment she was conceived [syn: Immaculate Conception,
       Immaculate Conception of the Virgin Mary]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "Immaculate Conception":
   Dei Mater, Deipara, Holy Mary, Mariolatry, Mariology,
   Mother of God, Notre Dame, Our Lady, Queen of Angels,
   Queen of Heaven, Regina Angelorum, Regina Coeli,
   Sancta Virgo Virginum, Stella Maris, Theotokos,
   Virgin Most Glorious, Virgin Most Merciful, Virgin Most Powerful,
   Virgin Most Renowned, Virgin Most Venerable, Virgin Most Wise,
   Virgo Clemens, Virgo Gloriosa, Virgo Potens, Virgo Praedicanda,
   Virgo Sapientissima, Virgo Sponsa Dei, Virgo Veneranda,
   mater dolorosa, the Blessed Virgin, the Sorrowful Mother,
   the Virgin, the Virgin Mary, the Virgin Mother