Whey or milk plasma is the liquid remaining after milk has been curdled and strained; it is a byproduct of cheese making with few commercial uses.

The whey protein separated from this mixture is often sold as a nutritional supplement. In addition, liquid whey contains lactose, vitamins, and minerals along with traces of fat.

As the most commonly used curdling agent is rennet, neither whey nor curds nor cheeses made therefrom automatically qualify as vegetarian foods. Vegetarians generally use lemon juice (or pure citric acid) to separate milk into curds and whey.