McDonald's M-shaped symbol is called the Golden Arches.
The arches originated in restaurant architecture rather than as a purely graphic logo. Architect Stanley Meston designed large yellow arches for McDonald's restaurants in the early 1950s. The arches were intended to make the buildings visible to passing motorists and to give the restaurants a distinctive appearance.
In 1962, Jim Schindler combined the two arches into an “M” design for McDonald's corporate identity. That mark gradually became more important than the buildings themselves and developed into one of the world's most recognizable commercial symbols.
The Golden Arches are sometimes casually called the “golden M,” but “Golden Arches” is the established name. The term refers to the branding symbol, not to the physical arches on every restaurant building.