Longman Dictionary of Contemporary English
Wiktionary
n. Milk that has been concentrated by evaporation, often sold canned.
WordNet
n. milk concentrated by evaporation
Wikipedia
Evaporated milk, known in some countries as unsweetened condensed milk, is a shelf-stable canned milk product with about 60% of the water removed from fresh milk. It differs from sweetened condensed milk, which contains added sugar. Sweetened condensed milk requires less processing since the added sugar inhibits bacterial growth.
The product takes up half the space of its nutritional equivalent in fresh milk. When the liquid product is mixed with a proportionate amount of water, evaporated milk becomes the rough equivalent of fresh milk. This makes evaporated milk attractive for shipping purposes as it can have a shelf life of months or even years, depending upon the fat and sugar content. This made evaporated milk very popular before refrigeration as a safe and reliable substitute for perishable fresh milk, which could be shipped easily to locations lacking the means of safe milk production or storage.
The process involves the evaporation of 60% of the water from the milk, followed by homogenization, canning, and heat-sterilization.