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

New particles and rare processes can be revealed through accelerator physics at or beyond the current energy frontiers, but this is not without risk. —Big Think, 5 Mar. 2026

A couple of outliers aside, even the most enthusiastic proponents of AGI don’t believe that the frontier AI models are capable of feeling. —Charles Yu, The Atlantic, 5 Mar. 2026

Billed as the most remote dude ranch in the Lower 48, Wyoming's Darwin Ranch really captures the Wild West frontier spirit with its isolated expanses and away-from-it-all appeal. —Evie Carrick, Travel + Leisure, 4 Mar. 2026

Turkey has drawn up plans to deal with a potential inflow of people fleeing ​the war in neighbouring Iran, Interior Minister Mustafa ‌Ciftci said today, with preparations including possible buffer zones along the frontier and tent camps. —Matt Nighswander, NBC news, 4 Mar. 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