: the yellow inner part of the egg of a bird or reptile containing stored food material for the developing young
Word History of YOLK
The word yolk goes back to Old English geoloca (a spelling that seems less strange when you know that ge- in Old English could stand for the sound /y/). Geoloca, in turn, is a derivative of geolu, the ancestor of our word yellow. This makes perfect sense given the color of a yolk, but changes in pronunciation over the course of centuries no longer make the relationship of the words obvious.
Medical Definition of YOLK
: material stored in an ovum that supplies food to the developing embryo and consists chiefly of proteins, lecithin, and cholesterol