frontier

noun

fron·​tier ˌfrən-ˈtir 

ˈfrən-ˌtir, 

frän-ˈtir, 

ˈfrän-ˌtir 

1

a

: a border between two countries

the frontier between Canada and the U.S.

b

obsolete : a stronghold on a frontier

2

a

: a region that forms the margin of settled or developed territory

an expedition to explore the western frontier

b

: the farthermost limits of knowledge or achievement in a particular subject

frontiers in immunology

c

: a line of division between different or opposed things

… the frontiers separating science and the humanities …

—R. W. Clark

d

: a new field for exploitative or developmental activity

… the brain lies before us as one of the last scientific frontiers here on earth.

—Robert Klitzman

frontier adjective

Synonyms of frontier

• border

Examples of frontier in a Sentence

the frontier between Canada and the U.S.

They were sent on an expedition to explore the western frontier.

Recent Examples on the Web

Leaders from media, frontier labs, industry and academia convened at Georgetown University to discuss governing frontier AI, according to a dispatch from WP Intelligence Reporter/Researcher Kendrick Frankel. — Benjamin Guggenheim, Washington Post, 18 Sep. 2026

And China, which has dismissed accusations of frontier model copying, has called on the US to listen to its business community or else risk putting the US at a further economic disadvantage. — Ashley Belanger, ArsTechnica, 18 Sep. 2026

Defense Minister Władysław Kosiniak-Kamysz said Tuesday that soldiers from an engineering regiment were reinforcing border guard infrastructure along the Polish-Ukrainian frontier. — Anna Noryskiewicz, CBS News, 18 Sep. 2026

On Sunday, two Russian drones struck near border crossings on the Polish-Ukrainian frontier. — Arkansas Online, 18 Sep. 2026

Word History

Etymology

Middle English fronter, from Anglo-French frountere, fronter, from front

First Known Use

15th century, in the meaning defined at sense 1a

Rhymes for frontier

• adhere
• ameer
• amir
• appear
• arrear
• astir
• austere
• career
• cashier
• cohere
• compere
• emir

Kids Definition

frontier

noun

fron·​tier ˌfrən-ˈti(ə)r  frän- 

1

: a border between two countries

2

a

: a region that forms the edge of the settled part of a country

b

: the outer limits of knowledge or achievement

the frontiers of science

frontier adjective