"frontiers" in the noun sense
1. frontier
a wilderness at the edge of a settled area of a country
"the individualism of the frontier in Andrew Jackson's day"
2. frontier
an international boundary or the area (often fortified) immediately inside the boundary
3. frontier
an undeveloped field of study a topic inviting research and development
"he worked at the frontier of brain science"
Source: WordNet® (An amazing lexical database of English)
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