The Collaborative International Dictionary of English v.0.48:

Yolk \Yolk\ (y[=o]lk or y[=o]k; 277), n. [OE. yolke, yelke,
   [yogh]olke, [yogh]elke, AS. geoloca, geoleca, fr. geolu
   yellow. See Yellow.] [Written also yelk.]
   [1913 Webster]
   1. The yellow part of an egg; the vitellus.
      [1913 Webster]

   2. (Zool.) An oily secretion which naturally covers the wool
      of sheep.
      [1913 Webster]

   Yolk cord (Zool.), a slender cord or duct which connects
      the yolk glands with the egg chambers in certain insects,
      as in the aphids.

   Yolk gland (Zool.), a special organ which secretes the yolk
      of the eggs in many turbellarians, and in some other
      invertebrates. See Illust. of Hermaphrodite in Appendix.


   Yolk sack (Anat.), the umbilical vesicle. See under
      Unbilical.
      [1913 Webster]

WordNet (r) 3.0 (2006):

yolk
    n 1: the yellow spherical part of an egg that is surrounded by
         the albumen [syn: egg yolk, yolk]
    2: nutritive material of an ovum stored for the nutrition of an
       embryo (especially the yellow mass of a bird or reptile egg)
       [syn: yolk, vitellus]