The western United States during its frontier period WordNet 3.0 & Open English WordNet

The American frontier, also known as the Old West, popularly known as the Wild West, encompasses the geography, history, folklore, and culture associated with the forward wave of American expansion in mainland North America that began with European colonial settlements in the early 17th century and ended with the admission of the last few contiguous western territories as states in 1912. Wikipedia

American westward expansion from 1600 to 1910 Wikipedia Disambiguation

Undeveloped territory of the United States c. 1607–1920 Wikidata