|Origin:||frontière, from front; FRONT1|
1 [countable] especially British EnglishSG
the border of a country
Lille is close to the frontier between France and Belgium.
on/at the frontier
Troops established a road block on the frontier.
frontier town/area/post etc (=a town etc on a frontier)
an area where people have never lived before, that not much is known about, especially in the western US before the 20th century:
a novel about a family's struggle on the American frontier
space, the final frontier