frontiers

'frontiers'

[frontier.n.01] as in: a wilderness at the edge of a settled area of a country

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Examples:
the individualism of the frontier in Andrew Jackson's day

'frontiers'

[frontier.n.02] as in: an international boundary or the area (often fortified) immediately inside the boundary

'frontiers'

[frontier.n.03] as in: an undeveloped field of study; a topic inviting research and development

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Examples:
he worked at the frontier of brain science