The phrase "Lower 48" became popular during the 20th century as a way to distinguish between the contiguous United States and the more remote states of Alaska and Hawaii. Hawaii became the 50th state of the union in 1959, while Alaska joined in 1959. Before this time, the term "Mainland United States" was commonly used to refer to the contiguous states, but the term "Lower 48" gradually became more popular as a more concise and descriptive way to make the distinction.
Despite the fact that the term "Lower 48" is commonly used, it is not an official designation or term used by the U.S. government or any official organization. It is simply a colloquial expression that has been widely adopted and recognized by the general public.