A white liquid derived from animals (on Earth, usually cows) and used as a nourishing drink.
A warm milk is Barclay's real drink of choice in Ten-Forward, according to Guinan. In one holodeck fantasy, he swaggered in for some kind of whiskey. Milk drew La Forge's scorn, but Guinan recommended it as a sleep aid. Aboard Voyager, milk is kept in a storage container after replication for easy access from the cooling unit.