fron‧tier / ˈfrʌntɪə $ frʌnˈtɪr / noun
1 [ countable ] especially British English the border of a country
frontier between/with
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 )
2 the 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
3 the frontiers of knowledge/physics etc the limits of what is known about something
push back the frontiers (= discover new things )
THESAURUS
border the official line that separates two countries, or the area close to this line : The town lies on the border between Chile and Argentina. | Strasbourg is very close to the German border. | border guards
frontier especially British English the border : They crossed the Libyan frontier into Egypt.
line the official line that separates states and counties in the US : His family lived across the state line in West Virginia.
boundary the line that marks the edge of an area of land that someone owns, or one of the parts of a country : The fence marks the boundary between the two properties. | The Mississippi River forms the boundary between Tennessee and Arkansas. | A road runs along the western boundary of the site.