A frontier is a political and geographical term describing the transitional zone between settled, governed territory and unknown or contested land, a concept that played a central role in colonial expansion across the Americas, Asia, and Australia. In the United States, historian Frederick Jackson Turner famously theorized this idea in his 1893 Frontier Thesis, arguing that the frontier experience was the defining force in shaping American democracy and national character, while Canada developed a parallel tradition through scholars like Harold Adams Innis. The word entered English from French in the 15th century meaning "borderland," but its specific use to describe the edge of settled territory is a distinctly North American development. In colonial Australia, the term was often linked to frontier violence, referring to "outside districts" where settlers clashed with Indigenous peoples, sometimes calling for protective forces like the Native Police. Meanwhile, Brazil developed its own unique frontier mythology around the Sertão—a vast, mysterious hinterland—where cattle raising expanded through wars against native peoples (the Barbarian Wars), later giving rise to the outlaw legend of the Cangaço after the devastating drought of 1877–1878. These varied interpretations show how the frontier concept adapted to different national contexts, shaping identities and policies worldwide.