Sweetened condensed milk is milk that has been simmered to evaporate most of its water, then mixed with sugar, producing a thick, shelf-stable dairy product. Gail Borden patented the condensing process in 1856 as a way to preserve milk without refrigeration.
It differs from evaporated milk, which also has water removed but contains no added sugar; sweetened condensed milk is sweeter, thicker, and more concentrated. To make it, milk is reduced by roughly 60% in volume, then combined with sugar at about a 2:1 milk-to-sugar ratio.
It's used in desserts and drinks worldwide: key lime pie and fudge in the US, dulce de leche in Latin America, and Vietnamese iced coffee and Thai iced tea in Southeast Asia. Variants exist made from goat's milk or soy milk for dietary needs.