Evaporated milk is whole, low-fat, or skim milk that has been evaporated to about one-half its volume. The milk is sealed in cans and heat sterilized. It can be reconstitued by adding to the milk an equal amount of water.
There are a few different ways to make evaporated milk if you aren't stocked up in the cupboard. Most recipes that call for evaporated milk are a sweet recipe (like fudge) and you are much better off slowly boiling the milk until it evaporates enough water off that it is half the volume. But for recipes like the above enchilada dip casserole, I just mix together dry milk and half the water. For instance, the recipe calls for a 5-1/3 oz can of evaporated milk; so I use these amounts to substitute the milk for that recipe:
1/4 cup + 1 TBLsp dry milk
1/2 cup + 1/8 cup water