The contiguous United States or officially the conterminous United States^[1] consists of the 48 adjoining U.S. states (plus the District of Columbia) on the continent of North America.^[2] The terms exclude the non-contiguous states of Alaska and Hawaii, and all other off-shore insular areas, such as American Samoa, U.S. Virgin Islands, Northern Mariana Islands, Guam and Puerto Rico.^[3][4] These differ from the related term continental United States which includes Alaska (also on the North American continent but separated from the 48 states by British Columbia, Canada) but excludes the Hawaiian Islands and unincorporated U.S. territories in the Caribbean.^[1][5]

The greatest distance (on a great circle route) entirely within the 48 contiguous states is 2,802 miles (4,509 km, between Florida and the State of Washington);^[6] the greatest north-south line is 1,650 miles (2,660 km).^[7]

Together, the 48 contiguous states and the District of Columbia occupy a combined area of 3,119,884.69 square miles (8,080,464.3 km2). Of this area, 2,959,064.44 square miles (7,663,941.7 km2) is contiguous land, composing 83.65% of total U.S. land area, similar to the area of Australia.^[8] Officially, 160,820.25 square miles (416,522.5 km2) of the contiguous United States is water area, composing 62.66% of the nation's total water area.

The contiguous United States would be placed 5th in the list of sovereign states and dependencies by area; the total area of the country, including Alaska and Hawaii, ranks fourth. Brazil is the only country that is larger in total area than the contiguous United States, but smaller than the entire United States, while Russia, and Canada, and China are the only three countries larger than both. The 2010 census population of this area was 306,675,006, comprising 99.33% of the nation's population, and a density of 103.639 inhabitants/sq mi (40.015/km2), compared to 87.264/sq mi (33.692/km2) for the nation as a whole.^[9]

While conterminous U.S. has the precise meaning of contiguous U.S. (both adjectives meaning "sharing a common boundary"), other terms commonly used to describe the 48 contiguous states have a greater degree of ambiguity.

Because Alaska is also part of North America, the term continental United States also includes that state, so the term is qualified with the explicit inclusion of Alaska to resolve any ambiguity.^{[3][10][11][12]} On May 14, 1959, the United States Board on Geographic Names issued the following definitions based partially on the reference in the Alaska Omnibus Bill, which defined the continental United States as "the 49 States on the North American Continent and the District of Columbia..." The Board reaffirmed these definitions on May 13, 1999.^[13] However, even before Alaska became a state, it was properly included within the continental U.S. due to being an incorporated territory.^[14]

Likewise, the term mainland United States is referred to any U.S. state located on the landmass continent of North America, which excludes Hawaii, off-shore insular areas, and nearby adjacent islands such as the Aleutian Islands (Alaska), San Juan Islands (Washington), and the Channel Islands (California).^[15]

CONUS, a technical term used by the U.S. Department of Defense, General Services Administration, NOAA/National Weather Service, and others, has been defined both as the continental United States, and as the 48 contiguous states.^[16][17] The District of Columbia is not always specifically mentioned as being part of CONUS.^[17]

OCONUS is derived from CONUS with O for outside added, thus referring to Outside of Continental United States (OCONUS).^[16][18]

The term lower 48 is also used to refer to the conterminous United States. The National Geographic style guide recommends the use of contiguous or conterminous United States instead of lower 48 when the 48 states are meant, unless used in the context of Alaska.^[19][20] Almost all of Hawaii is in fact south of the southernmost point of the conterminous United States, in Florida.

During World War II, the first four numbered Air Forces of the United States Army Air Forces (USAAF) were said to be assigned to the Zone of the Interior by the American military organizations of the time—the future states of Alaska and Hawaii, then each only organized incorporated territories of the Union, were respectively covered by the Eleventh Air Force and Seventh Air Force during the war.

Alaskans, Hawaiians, and off-shore territories have unique labels for the contiguous United States because of their own locations relative to them.

Alaska became the 49th state of the United States on January 3, 1959. Alaska is on the northwest end of the North American continent, but separated from the rest of the United States West Coast by the Canadian province of British Columbia. The term Lower 48 has, for many years, been a common Alaskan equivalent for "contiguous United States";^[21][22] today, more Alaskans use the term "Outside",^[23] though a few persons may use "Outside" to refer to any location not within Alaska.^[24]

Hawaii became the 50th state of the United States on August 21, 1959. It is the southernmost and so far, the latest state to join the Union. Not part of any continent, Hawaii is located in the Pacific Ocean, about 2,200 miles (3,500 km) from North America and almost halfway to Asia. In Hawaii and overseas American territories, for instance, the terms the Mainland or U.S. Mainland are often used to refer to the 49 states in North America.^[25][26]

Puerto Rico is an unincorporated territory of the United States located in the northeast Caribbean Sea, approximately 1,000 miles (1,600 km) southeast of Miami, Florida. Puerto Ricans born in Puerto Rico are U.S. citizens and are free to move to the United States. A Stateside Puerto Rican is a term for residents in a U.S. state who were born in or trace family ancestry to Puerto Rico.^[27]

Apart from off-shore U.S. islands, a few continental portions of the contiguous U.S. are accessible by road only by traveling through Canada. Point Roberts, Washington; Elm Point, Minnesota; and the Northwest Angle in Minnesota are three such places. Alburgh, Vermont, is not directly connected by land, but is accessible by road via bridges from within Vermont and from New York.^[28]

The 48 contiguous United States are: