The white part of an egg, also known as the albumen. It contains virtually no fat and is high in protein.
When egg whites are whisked, air is trapped in them, and remains trapped when the whisked egg whites are folded into meringues, mousses or soufflés and baked. Egg whites will whisk to a greater volume when they're at room temperature than when chilled, so always remove them from the fridge to bring them up to room temperature before you start cooking. When heated, the proteins in egg whites will combine and coagulate, which is essential for making cakes, pancakes and batter puddings. Egg whites also act as a stabiliser: the protein in egg whites will minimise the chance of ice crystals forming in a sorbet as it freezes.
Vegans should be aware that egg whites are sometimes used as a fining agent in winemaking.