A Big Mac is a popular hamburger sold by the fast-food chain McDonald's. It consists of two beef patties, special sauce, lettuce, cheese, pickles, and onions, all served on a sesame seed bun. The sandwich is known for its distinctive flavor and is often considered a symbol of American fast food culture. Introduced in 1968, the Big Mac has become one of McDonald's signature items. It is available in many countries around the world, often with slight variations to cater to local tastes. The sandwich is also featured in the Big Mac Index, an informal measure of purchasing power parity between nations.