/jəʊk/
Noun, plural yolks
1. Material stored in an ovum that supplies food to the developing embryo and consists chiefly of proteins, lecithin, and cholesterol. (merriam-webster.com)
2. The yellow internal part of a bird’s egg, which is surrounded by the white, is rich in protein and fat, and nourishes the developing embryo (Google Dictionary); The yellow, spherical part of an egg that is surrounded by the white albumen, and serves as nutriment for the growing young. (wiktionary.org)
3. Also called Deutoplasm, the nutritive material of an egg, used as food by a developing, embryonic animal. (global.britannica.com)
4. 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. (freedictionary.com)
5. The grease in a sheep’s fleece. (wiktionary.org)
Word origin: Old English geolca, geoloca “yolk,” literally “the yellow part,” from geolu “yellow.”