From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
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]