The Collaborative International Dictionary of English v.0.48:

Bonanza \Bo*nan"za\, n. [Sp., prop. calm., fair weather,
   prosperity, fr. L. bonus good.]
   In mining, a rich mine or vein of silver or gold; hence,
   anything which is a mine of wealth or yields a large income.
   [Colloq. U. S.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

bonanza
    n 1: an especially rich vein of precious ore
    2: a sudden happening that brings good fortune (as a sudden
       opportunity to make money); "the demand for testing has
       created a boom for those unregulated laboratories where boxes
       of specimen jars are processed like an assembly line" [syn:
       boom, bonanza, gold rush, gravy, godsend, manna
       from heaven, windfall, bunce]