In American history the frontier is the process of settlement of new lands in the West, plus the geographical zone of settlement, together with the impact on the frontiersmen and the nation at large. As pioneers moved west they changed their customs, behavior and values and became more "American"; Frederick Jackson Turner called this "the significance of the frontier," Turner argued in 1893, one change was that unlimited free land in the zone was available and thus offered the psychological sense of unlimited opportunity, which in turn had many consequences, such as optimism, future orientation, shedding of restraints due to land scarcity, and wastefulness of natural resources. See Frontier Thesis
Throughout American history, the expansion of settlement was largely from the east to the west, and thus the frontier is often identified with "the west." In New England, it moved north, so that Maine and Vermont had frontier characteristics/
See also: Colonial America
Before 1700, the frontier was any part of the forested interior of the continent beyond the 10 to 50 mile fringe of existing settlements along the coast.
English, French, Spanish and Dutch patterns of expansion and settlement were quite different. Only a few thousand French migrated to Canada; these habitants settled in villages along the St. Lawrence river, building communities that remained stable for long stretches; they did not leapfrog west the way the Americans did. Although French fur traders ranged widely through the Great Lakes region they seldom settle down. Likewise, the Dutch set up fur trading posts in the Hudson river valley, followed by large grants of land to rich landowning patroons who brought in tenant farmers who created compact, permanent villages. They did not push westward. 
In contrast, the English colonies generally pursued a more systematic policy of widespread settlement of the New World for cultivation and exploitation of the land, a practice that required the application of legal property rights to the new conditions. The typical English settlements were quite compact and small--under a square mile. Conflict with the Native Americans arose out of political issues, viz. who would rule. Early frontier areas east of the Appalachian Mountains included the Connecticut River valley. The French and Indian Wars of the 1760s resulted in a complete victory for the British, who took over the lands west to the Mississippi River. By the early 1770s Americans were moving across the Appalachians into western Pennsylvania, Kentucky, Tennessee and Ohio.
Early National Era, 1770s-1830s
After the Revolutionary war ended (1781), the Americans in large numbers poured into the west. In some areas they had to battle the Indian tribes. No Indians lived in Kentucky but they sent raiding parties to stop the newcomers, like Abe Lincoln's grandfather (who was scalped in 1786 near Louisville.) The War of 1812 marked the final confrontation between major Indian forces trying to stop the advance, with British aid. American frontier militiamen under Andrew Jackson defeated the Creeks and opened the Southwest, while militia under William Henry Harrison defeated the Indian-British alliance at a battle in Canada. The death in battle of the Indian leader Tecumseh dissolved the coalition of hostile Indian tribes. In general the frontiersmen battled the Indians with little help from the U.S. Army or the federal government. Indeed, the regular army set up a line of forts designed to keep the Indians and settlers apart.
As settlers poured in the new areas went through the territorial stage and became states, typically dropping the legalistic practices favored by eastern upper classes, and adopting more democracy and more egalitarianism.
Later frontier, 1830s-1890
see Oregon Trail
Debate on Turner's interpretation
see Frontier Thesis Kearns (1998) examines the environmentalism of William Cronon and Donald Worster, and the revisionist moralism of Richard White and Patricia Limerick, who reject Turner as too favorable toward white men.
Since the 1970s the "new western historians" have attacjed Turner's model. They have condemned the frontier thesis for its Euro-centric and racist assumptions, ridiculing Turner for his depiction of enlightened whites and savage natives and for discounting Indian agency. They assailed Turner's argument that the frontier created America character and ideology by revealing how Americans drew upon European antecedents and their own experiences in urban settings. Other maintain that the frontier was neither especially democratic nor equal. Nevertheless the Turnerians have counterattacked, saying that the critics confuse 21st century moralistic sensiblities with historical reality. Turner's model, they note, was not about the Indians (who Turner wrote about elsewhere), but rather about the impact of the frontier on the frontiersmen and all Americans. Agreeing that the frontiersmen did not jettison all European ideas, the Turnerians argue they decisively remoulded and reshaped them to meet American conditions. The critics who suggest that democracy emerged from boss-ridden urban machines like Tammany Hall have surely misunderstood what American democracy means.
- Billington, Ray Allen. America's Frontier Heritage (1984), an analysis of the frontier experience from perspective of social sciences and historiography
- Billington, Ray Allen. Westward Expansion: A History of the American Frontier (1952 and later editions), the most detailed textbook, with highly detailed annotated bibliographies
- Bouton, Terry. "The New and (Somewhat) Improved Frontier Thesis," Reviews in American History Volume 35, Number 4, December 2007 online at Project Muse
- Hine, Robert V. and John Mack Faragher. Frontiers: A Short History of the American West (2008), recent textbook excerpt and text search
- Johnson, Charles A. The Frontier Camp Meeting: Religion's Harvest Time (1955) online edition
- Kearns, Gerry. "The Virtuous Circle of Facts and Values in the New Western History," Annals of the Association of American Geographers (1998) 88 (3), 377–409.
- Lamar, Howard R. ed. The New Encyclopedia of the American West (1998), 1000+ pages of articles by scholars
- Limerick, Patricia Nelson. The Legacy of Conquest: The Unbroken Past of the American West (1987) influential attack on Turnerian models.
- Milner, Clyde A., II ed. Major Problems in the History of the American West 2nd ed (1997), primary sources and essays by scholars
- Nichols, Roger L. ed. American Frontier and Western Issues: An Historiographical Review (1986) essays by 14 scholars
- Paxson, Frederic, History of the American Frontier, 1763-1893 (1924), old textbook by Pulitzer Prize winner
- Slotkin, Richard, Regeneration Through Violence: The Mythology of the American Frontier, 1600-1860 (2000), University of Oklahoma Press
- Turner, Frederick Jackson. "The Frontier In American History" (1893)
- Billington, Ray Allen. Land of Savagery / Land of Promise: The European Image of the American Frontier in the Nineteenth Century (1981)
- Griffin, Patrick. American Leviathan: Empire, Nation, and Revolutionary Frontier. (2007). 368 pp.
- Unruh, John D., Jr. The Plains Across: The Overland Emigrants and the Trans-Mississippi West, 1840-1860. (1979), the outstanding scholarly history. excerpt and text search
- Wrobel, David M. "Exceptionalism and Globalism: Travel Writers and the Nineteenth-Century American West," Historian, 68 (Fall 2006), 431–60.
- French settlement in these areas was limited to a few very small villages such as Kaskaskia. Clarence Walworth Alvord, The Illinois Country 1673-1818 (1918)
- Arthur G. Adams, The Hudson Through the Years (1996); Sung Bok Kim, Landlord and Tenant in Colonial New York: Manorial Society, 1664-1775 (1987)
- Allan Kulikoff, From British Peasants to Colonial American Farmers (2000)