Idaho is a mostly mountainous state, with an area larger than all of New England. It is landlocked, surrounded by the states of Washington, Oregon, Nevada, Utah, Wyoming, Montana and the Canadian Province of British Columbia. However, the network of dams and locks on the Columbia River and Snake River make the city of Lewiston the farthest inland seaport on the west coast of the continental United States.

According to the US Census Bureau, in 2009 the population for Idaho was estimated to be 1,545,801. The state's postal abbreviation is ID. Idaho's nickname is the Gem State because nearly every known gem has been found there. In addition, Idaho is one of only two places in the world where star garnets can be found (the other is the Himalaya Mountains, in India), and is the only place six pointed star garnets have been found. The state motto is Esto Perpetua (Latin for "Let it be forever").

Idaho is the 14th largest state by land area.

Idaho was possibly named as the result of a hoax (the so-called "Idahoax"), although this is disputed. The exact origin of the name remains a mystery. In the early 1860s, when the United States Congress was considering organizing a new territory in the Rocky Mountains, eccentric lobbyist George M. Willing suggested the name "Idaho," which he claimed was derived from a Shoshone language term meaning "the sun comes from the mountains" or "gem of the mountains". Willing later claimed that he had made up the name himself. Congress ultimately decided to name the area Colorado Territory when it was created in February 1861. Thinking they would get a jump on the name, locals named a community in Colorado "Idaho Springs".

However, the name "Idaho" did not go away. The same year Congress created Colorado Territory, a county called Idaho County was created in eastern Washington Territory. The county was named after a steamship named Idaho, which was launched on the Columbia River in 1860. It is unclear whether the steamship was named before or after Willing's claim was revealed. Regardless, a portion of Washington Territory, including Idaho County, was used to create Idaho Territory in 1863.

Despite this lack of evidence for the origin of the name, many textbooks well into the 20th century repeated as fact Willing's account that the name "Idaho" derived from the Shoshone term "ee-da-how".

The name "Idaho" may be derived from the Plains Apache word "ídaahę́" which means "enemy." The Comanches used this word to refer to the Idaho Territory.

An excerpt from an Idaho History Textbook:

Chief Joseph Seltice, of the Coeur d'Alene Tribal Nation, posits another possible origin of the name. In his history of the tribe, Saga of the Coeur d'Alene Indians, he writes:

Some sources claim that the name "Idaho" comes from an Indian word, "Ee-dah-how," meaning "Gem of the Mountains." This expression may have come from some other Tribe, and it would have a different meaning for them than it would for the Coeur d'Alenes.As the Coeur d'Alenes understood the word "Idaho," it would be more correctly pronounced "Ah-d'Hoo." It means "greetings by surprise," indicating friendship, but surprise.The first syllable conveys to the mind, "All are welcome, from wherever you come; but keep the friendly peace. We welcome you with out-stretched arms, and this entitles us to permanent friendship."The last syllable is a surprise and exclamation point. The expression means that all are welcome, "though we are surprised to see so many different strangers. The first dawn of day welcomes you as the sun rises." This expression was used by many of the Coeur d'Alenes on the Bitterroot Mountains to greet all who come.So to all who read these words: "Welcome, with open arms! We're just surprised that there are so many of you!"