Curdled milk is not only an unappetizing sight but it can also be an indication of milk spoilage. There are several synonyms that are often used to describe this unpleasant condition. Sour milk is a common synonym for curdled milk which indicates the presence of lactic acid that makes milk taste sour and can even make it clump and curdle. Another synonym for curdled milk is spoiled milk which is an indication of milk that has gone bad. Lastly, lumpy milk is another term used to describe curdled milk which often separates into lumpy curds and a watery whey.