Albumen is the clear, thick liquid inside an egg that turns white and firm when you cook it. Most people call it egg white, but scientists and cooks use the term albumen. When you crack an egg into a bowl, the albumen is the gooey, transparent part surrounding the yellow yolk.

Albumen serves as food and protection for a developing chick. It's mostly water and protein, which is why it changes texture so dramatically when heated: the proteins bond together and transform from slippery liquid to solid white. This same protein makes albumen incredibly useful in cooking. Bakers whip it into stiff peaks for meringues, use it to make angel food cake light and fluffy, or brush it on pastries to create a shiny glaze.

In medicine and science, albumin (a related word) can refer to protein found in blood, and albumen can also mean protein-rich material in plant seeds. But when most people use the word albumen, they're talking about eggs.