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"