The Collaborative International Dictionary of English v.0.48:

Gravy \Gra"vy\, n.; pl. Gravies. [OE. greavie; prob. fr.
   greaves, graves, the sediment of melted tallow. See
   Greaves.]
   1. The juice or other liquid matter that drips from flesh in
      cooking, made into a dressing for the food when served up.
      [1913 Webster]

   2. Liquid dressing for meat, fish, vegetables, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gravy
    n 1: a sauce made by adding stock, flour, or other ingredients
         to the juice and fat that drips from cooking meats
    2: the seasoned but not thickened juices that drip from cooking
       meats; often a little water is added [syn: gravy, pan
       gravy]
    3: 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]