yolk

American  

[yohk, yohlk] / yoʊk, yoʊlk /

noun

1. the yellow and principal substance of an egg, as distinguished from the white.

2. Embryology. the part of the contents of the egg of an animal that enters directly into the formation of the embryo, together with any material that nourishes the embryo during its formation.

3. the essential part; the inner core.

4. a natural grease exuded from the skin of sheep.

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yolk British  

/ jəʊk /

noun

1. the substance in an animal ovum consisting of protein and fat that nourishes the developing embryo

2. a greasy substance secreted by the skin of a sheep and present in the fleece

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yolk Scientific  

/ yōk /

1. The yellow internal part of the egg of a bird or reptile. The yolk is surrounded by the albumen and supplies food to the developing young.

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Other Word Forms

• yolked adjective
• yolkless adjective
• yolky adjective

Etymology

Origin of yolk

before 1000; Middle English yolke, yelke, Old English geoloca, derivative of geolu yellow

Explanation

A yolk is the inner, yellow part of an egg. If you order eggs "sunny side up," those "suns" are the egg yolks. When a bird lays an egg that's fertilized and begins to grow into a baby bird, the yolk is the substance that nourishes the bird embryo. The eggs we eat have yolks too, but since they are unfertilized, these protein-rich yolks can be beaten into omelettes or mixed into cake batter. The Old English root of yolk, geolca, literally means "the yellow part."

Example Sentences

This work introduces a new framework for understanding cell division in large, yolk rich embryos and could apply to many egg laying species.

From Science Daily • Feb. 28, 2026

Battling strong currents, Ralph Pace captured this image of a swell shark egg case, revealing a glowing embryo, complete with gill slits and a yolk sac.

From BBC • Oct. 14, 2025

Every bite is a mash-up of textures and flavors — rich, tangy, savory, with a touch of runny yolk magic, which doubles as the “sauce” here.

From Salon • Sep. 16, 2025

Eventually, membrane or no, there will be some gas exchange, causing the egg white to become more watery and leak into the yolk, ultimately breaking the yolk.

From Slate • Apr. 19, 2025

Twig didn’t respond, just kept cracking invisible eggs over my head and running streams of fingertip yolk through my hair.

From "The Science of Breakable Things" by Tae Keller