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]