The Collaborative International Dictionary of English v.0.48:

El Dorado \El` Do*ra"do\, pl. El Doradoes. [Sp., lit., the
   gilt (sc. land); el the + dorado gilt, p. p. of dorare to
   gild. Cf. Dorado.]
   1. A name given by the Spaniards in the 16th century to an
      imaginary country in the interior of South America,
      reputed to abound in gold and precious stones.
      [1913 Webster]

   2. Any region of fabulous wealth; exceeding richness.
      [1913 Webster]

            The whole comedy is a sort of El Dorado of wit. --T.
                                                  Moore.
      [1913 Webster]

WordNet (r) 3.0 (2006):

El Dorado
    n 1: an imaginary place of great wealth and opportunity; sought
         in South America by 16th-century explorers [syn: El
         Dorado, eldorado]