Big Mac became the name of McDonald's 1968 national burger launch.
The Big Mac was created by Jim Delligatti, a McDonald's franchisee who operated restaurants in the Pittsburgh area. It used two beef patties, a middle bun section, lettuce, cheese, pickles, onions, and sauce, giving the sandwich a larger and more distinctive format than McDonald's standard hamburger.
The sandwich was introduced nationally in 1968 after regional testing. Its name became one of the strongest product brands in fast-food history and later helped McDonald's create a recognizable vocabulary around menu items.
The Quarter Pounder is another major McDonald's burger, but it was introduced earlier, in 1971. The McDouble and Big N' Tasty arrived much later, so neither is the answer to the 1968 launch question.