The Collaborative International Dictionary of English v.0.48:

Spay \Spay\, v. t. [imp. & p. p. Spayed; p. pr. & vb. n.
   Spaying.] [Cf. Armor. spac'hein, spaza to geld, W. dyspaddu
   to geld, L. spado a eunuch, Gr. ?.]
   To remove or extirpate the ovaries of, as a sow or a bitch;
   to castrate (a female animal).
   [1913 Webster]

WordNet (r) 3.0 (2006):

spaying
    n 1: neutering a female by removing the ovaries