A casserole is a large, deep baking dish that can be used both in the oven and as a serving dish. Casserole is also what you call the food baked inside it, which is often a complete, gooey, one-dish meal.
You can use the word casserole for a ceramic, glass, or metal baking dish that goes right from the oven to the table, and you can also call the food that's cooked in it a casserole. Many casseroles are thick and warm comfort food, baked by thoughtful people in aprons. Cream of mushroom soup is a popular ingredient in a casserole. In French, casserole means "sauce pan," from the Latin root cattia, "pan" or "vessel."