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

Extropic has already begun shipping an early version of its thermodynamic computing platform to select partners, including frontier AI labs. — Rob Toews, Forbes.com, 22 June 2026

The Border Commandos, who control villages and smuggle cocaine along parts of Colombia's frontier with Ecuador, number about 1,000 fighters, thus only 10% of the group actually turned in weapons. — John Otis, NPR, 21 June 2026

Supporters call 3-D printing the new frontier in gun safety, while critics warn of censorship, data privacy risks and battles over Americans’ rights to build their own firearms. — David A. Lieb, Los Angeles Times, 20 June 2026

The frontier region had seen a rise in vaccine hesitancy for some time, said Sydnee Lyons, the health department's public information officer. — ABC News, 20 June 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