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

Once ladled into low bowls, each serving got a single glossy egg yolk and a small drift of fresh lemon zest.

From Salon • May 12, 2026

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

A yolk giving way under the edge of a knife.

From Salon • Oct. 23, 2025

"When they are small, they kind of look like an elephant squished into a little yolk sack."

From Science Daily • Oct. 16, 2025

This time she was long at labor and when Wang Lung came home at evening he found his father standing at the door and laughing and saying, “An egg with a double yolk this time!”

From "The Good Earth" by Pearl S. Buck