The Collaborative International Dictionary of English v.0.48:

Whey \Whey\, n. [AS. hw[ae]g; cf. D. wei, hui, Fries. weye, LG.
   wey, waje. ]
   The serum, or watery part, of milk, separated from the more
   thick or coagulable part, esp. in the process of making
   cheese.

   Note: In this process, the thick part is called curd, and
         the thin part whey.
         [1913 Webster]

WordNet (r) 3.0 (2006):

whey
    n 1: the serum or watery part of milk that is separated from the
         curd in making cheese [syn: whey, milk whey]
    2: watery part of milk produced when raw milk sours and
       coagulates; "Little Miss Muffet sat on a tuffet eating some
       curds and whey"

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "whey":
   beverage, blood, butter, buttermilk, certified milk, cheese,
   condensed milk, cream, dairy products, drink, fluid, fluid extract,
   fluid mechanics, ghee, half-and-half, heavy cream, hydraulics,
   hydrogeology, juice, latex, light cream, liquid, liquid extract,
   liquor, margarine, milk, nonfat dry milk, oleo, oleomargarine,
   raw milk, sap, semiliquid, skim milk, sour cream, water,
   whipping cream, yogurt