frontiers

noun
  1. 1

    The part of a country which borders or faces another country or unsettled region

  2. 2

    The most advanced or recent version of something; leading edge.

    โ€œthe frontier of civilizationโ€

  3. 3

    An outwork of a fortification.

Source: https://en.wiktionary.org/wiki/frontier, https://en.wiktionary.org/wiki/frontiers