The Collaborative International Dictionary of English v.0.48:

Madonna \Ma*don"na\, n. [It. madonna my lady. See Dame,
   Donna, and cf. Madame, Monkey.]
   1. My lady; -- a term of address in Italian formerly used as
      the equivalent of Madame, but for which Signora is now
      substituted. Sometimes introduced into English. --Shak.
      [1913 Webster]

   2. [pl. Madonnas (n[.a]z).] A picture of the Virgin Mary
      (usually with the babe).
      [1913 Webster]

            The Italian painters are noted for drawing the
            Madonnas by their own wives or mistresses. --Rymer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Madonna
    n 1: the mother of Jesus; Christians refer to her as the Virgin
         Mary; she is especially honored by Roman Catholics [syn:
         Mary, Virgin Mary, The Virgin, Blessed Virgin,
         Madonna]
    2: United States pop singer and sex symbol during the 1980s
       (born in 1958) [syn: Madonna, Madonna Louise Ciccone]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "Madonna":
   Dei Mater, Deipara, Holy Mary, Immaculate Conception, 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