yolk

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The yellow, usually spherical portion of an egg of a bird or reptile, surrounded by the albumen and serving as nutriment for the developing young.
2. n. A corresponding portion of the egg of other animals, consisting of protein and fat that serve as the primary source of nourishment for the early embryo and protoplasmic substances from which the embryo develops.
3. n. A greasy substance found in unprocessed sheep's wool.

Century Dictionary and Cyclopedia

1. n. 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. In holoblastic ova, which are usually of minute or microscopic size, the whole content of the cellwall is yolk which undergoes complete segmentation, and is therefore formative or germinal vitellus, or morpholecithus. In large meroblastic eggs, however, such as those we eat of various birds and reptiles, the true germyolk forms only the nucleus and a relatively small part of the whole yolk-ball, which then consists mainly of food-yolk or tropholecithus. This is the yolk of ordinary language, forming a relatively large ball of usually yellow and minutely granular substance which floats in a mass of white or colorless albumen, inclosed in a delicate pellicle, or vitelline membrane, and is steadied or stayed in position by certain strands of stringy albumen forming the chalazæ. The quautity of germ-and of food-yolk relatively to each other and also to the amount of white varies much in different eggs, as does also the relative position of the two kinds of yolk. (See ectolecithal, centrolecithal.) In the largest eggs, as of birds, the great bulk results from the copiousness of the white and of the food-yolk, and the germ-yolk appears only at a point on the surface of the latter, where it forms the so-called tread or cicatricula. Some eggs contain more than one yolk, but this is rare and anomalous. See egg, ovum, and vitellus; also segmentation of the vitellus (under segmentation), and cuts under gastrulation.
2. n. The vitellus, a part of the seed of plants, so named from its supposed analogy with the yolk of an egg.
3. n. The greasy sebaceous secretion or unctuous substance from the skin of the sheep, which renders the fleece soft and pliable; wool-oil.
4. See yoke.

Wiktionary

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

GNU Webster's 1913

1. n. The yellow part of an egg; the vitellus.
2. n. (Zoöl.) An oily secretion which naturally covers the wool of sheep.

WordNet 3.0

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