It is undeveloped region that is open to settlement; open border.It means the land or territory that forms the furthest extent of a country's settled or inhabited regions. Often frontiers. the limit of knowledge or the most advanced achievement in a particular field: the frontiers of physics.
<h2>closed frontier:</h2>
In 1890, the Census Bureau broadcast the closure of the frontier, meaning that in the west there was no apparent tracts of land without settlers. This news was a distinguished event in American history; the frontier represented danger because of the Natives who lived in the region but also freedom and opportunity.
