The Contiguous United States or the Lower 48 are the forty-eight states which form a single large geographical area. From 1912, when Arizona became the 48th state, until 1959, when Alaska became the 49th, this region included all the states.
The term excludes the two states which have no border with the contiguous region, Alaska and Hawaii. It also excludes US territories such as Puerto Rico and various Pacific and Caribbean islands which do not have status as states. (Washington, D.C. is not a state, but is contiguous with two of the lower 48 states.) The USA article lists all 50 states and the most important territories.