A terrine can refer to an earthenware casserole or to the dish made in it.
Terrine (the tool) is a covered glazed earthenware used to cook the terrine meatloaf.
Terrine (the food) is a French forcemeat loaf (meat is ground or finely chopped). It is placed into the covered terrine, and is cooked in a baine marie. When the dish cools, it is pressed firmly to release air pockets trapped in the terrine.