whey -
The liquid remaining after milk has been curdled and strained in the process of making cheese.
Wikipedia
Whey or milk plasma is the liquid remaining after milk has been curdled and strained. It is a by-product of the manufacture of cheese or casein and has several commercial uses. Sweet whey is manufactured during the making of rennet types of hard cheese like Cheddar or Swiss.
OmegaWiki Dictionary
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whey The watery liquid that separates from the curd when the milk is clotted, as in making cheese.