A cheeseburger is a hamburger topped with cheese. Traditionally, the slice of cheese is placed on top of the meat patty, but the burger can include many variations in structure, ingredients, and composition. The term itself is a portmanteau of the words "cheese" and "hamburger." The cheese is usually added to the cooking hamburger patty shortly before the patty is completely cooked which allows the cheese to melt. Cheeseburgers are often served with lettuce, tomato, onion, pickles, mustard, mayonnaise, ketchup, and occasionally bacon.