Idaho ( (listen)) is a state in the Pacific Northwest region of the United States. It borders the state of Montana to the east and northeast, Wyoming to the east, Nevada and Utah to the south, and Washington and Oregon to the west. To the north, it shares a small portion of the Canadian border with the province of British Columbia. With a population of approximately 1.8 million and an area of 83,570 square miles (216,400 km2), Idaho is the 14th largest, the 13th least populous and the 7th least densely populated of the 50 U.S. states. The state's capital and largest city is Boise.

For thousands of years Idaho has been inhabited by Native American peoples. In the early 19th century, Idaho was considered part of the Oregon Country, an area disputed between the United States and the British Empire. It officially became U.S. territory with the signing of the Oregon Treaty of 1846, but a separate Idaho Territory was not organized until 1863, instead being included for periods in Oregon Territory and Washington Territory. Idaho was eventually admitted to the Union on July 3, 1890, becoming the 43rd state.

Forming part of the Pacific Northwest (and the associated Cascadia bioregion), Idaho is divided into several distinct geographic and climatic regions. The state's north, the relatively isolated Idaho Panhandle, is closely linked with Eastern Washington with which it shares the Pacific Time Zone—the rest of the state uses the Mountain Time Zone. The state's south includes the Snake River Plain (which has most of the population and agricultural land). The state's southeast incorporates part of the Great Basin. Idaho is quite mountainous, and contains several stretches of the Rocky Mountains. The United States Forest Service holds about 38% of Idaho's land, the highest proportion of any state.Industries significant for the state economy include manufacturing, agriculture, mining, forestry, and tourism. A number of science and technology firms are either headquartered in Idaho or have factories there, and the state also contains the Idaho National Laboratory, which is the country's largest Department of Energy facility. Idaho's agricultural sector supplies many products, but the state is best known for its potato crop, which comprises around one-third of the nationwide yield. The official state nickname is the "Gem State", which references Idaho's natural beauty.