Real Dictionary

whey
uncountable noun: Whey is the watery part of sour milk that is separated from the thick part called curds, for example when you are making cheese.

whey in American English
the thin, watery part of milk, that separates from the thicker part (curds) after coagulation, as in cheese making
noun: a milk serum, separating as liquid from the curd after coagulation, as in cheese making

whey in British English
noun: the watery liquid that separates from the curd when the milk is clotted, as in making cheese