yolk

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The portion of the egg of egg-laying vertebrates, such as reptiles and birds, and of certain invertebrates that consists chiefly of protein and fat and serves as the primary source of nourishment for the early embryo.
• noun This portion of the egg of a bird, especially a chicken, which is large, yellow, and surrounded by albumen.
• noun A greasy substance found in unprocessed sheep's wool, which is refined to make lanolin.

from The Century Dictionary.

• See yoke.
• noun The yellow and principal substance of an egg, as distinguished from the white; that protoplasmic content of the ovum of any animal which forms the embryo in germination, with or without some additional substance which serves to nourish the embryo during its formation, as distinguished from a mass of albumen which may surround it, and from the egg-pod or shell which incloses the whole; the vitellus, whether formative wholly or in part.
• noun The vitellus, a part of the seed of plants, so named from its supposed analogy with the yolk of an egg.
• noun The greasy sebaceous secretion or unctuous substance from the skin of the sheep, which renders the fleece soft and pliable; wool-oil.

from the GNU version of the Collaborative International Dictionary of English.

• noun The yellow part of an egg; the vitellus.
• noun (Zoöl.) An oily secretion which naturally covers the wool of sheep.
• noun (Zoöl.) a slender cord or duct which connects the yolk glands with the egg chambers in certain insects, as in the aphids.
• noun (Zoöl.) a special organ which secretes the yolk of the eggs in many turbellarians, and in some other invertebrates. See Illust. of Hermaphrodite in Appendix.
• noun (Anat.) the umbilical vesicle. See under Unbilical.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The yellow, spherical part of an egg that is surrounded by the white albumen, and serves as nutriment for the growing young.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the yellow spherical part of an egg that is surrounded by the albumen
• noun nutritive material of an ovum stored for the nutrition of an embryo (especially the yellow mass of a bird or reptile egg)