That part of a country which fronts or faces another country or an unsettled region; the marches; the border, confine, or extreme part of a country, bordering on another country; the border of the settled and cultivated part of a country; as, the frontier of civilization
A massively scaleable distributed computing platform that draws on the otherwise unused computational capacity of computers connected to the Internet
an international boundary or the area (often fortified) immediately inside the boundary a wilderness at the edge of a settled area of a country; "the individualism of the frontier in Andrew Jackson's day
When you are talking about the western part of America before the twentieth century, you use frontier to refer to the area beyond the part settled by Europeans. a far-flung outpost on the frontier