Meaning

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)

originFrom Middle English yolke, yelke, ȝolke, ȝelke, from Old English ġeolca, ġeoloca, ġioleca (“the yellow part, yolk”), derived from Old English ġeolu (“yellow”) + -ca (diminutive suffix).

Etymology from Wiktionary, CC BY-SA 3.0