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]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "gravy":
   Colbert, Smitane, Soubise, Trinkgeld, allemande, bonus,
   bonus system, bounty, bourguignonne, bribe, brown sauce,
   buried treasure, consideration, cream sauce, discovery, donative,
   double time, egg sauce, espagnole, fee, find, finding, foundling,
   fringe benefit, gratuity, grease, honorarium, incentive pay,
   inducement, lagniappe, largess, liberality, marinara, mole,
   overtime pay, palm oil, paprika sauce, pepper sauce, perks,
   perquisite, perquisites, poulette, pourboire, premium, roux, salve,
   shallot sauce, solatium, something extra, sportula, sweetener, tip,
   treasure trove, trouvaille, trove, waifs, waifs and strays,
   windfall, windfall money, windfall profit