In U.S. History 19th Century use of the term American Frontier refers to?

The term "American Frontier" in 19th-century U.S. history refers to the expanding borderlands of American settlement and exploration, particularly as settlers moved westward across North America. This concept embodies the idea of manifest destiny, where Americans believed it was their divine right to expand across the continent. The frontier was characterized by the interaction between settlers and Native American tribes, as well as significant cultural, economic, and social changes. It played a crucial role in shaping American identity and ideals of freedom and opportunity.