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

What's actually happening is that the production frontier of business has moved. —Gabriel Alin Zainescu, Forbes.com, 10 May 2026

The airport said the majority of the passengers have since departed Denver on a new frontier flight. —Jack Lowenstein, CBS News, 9 May 2026

He's fought orcs from horseback in Middle-earth, explored the final frontier on the Starship Enterprise, and wielded dual machine guns in the Marvel Cinematic Universe, but now Karl Urban is really in the thick of it. —James Grebey, Time, 8 May 2026

Channel Islands National Park, California A throwback to old California, the five-island park preserves paleolithic digs, frontier-era ranches and relics of Spanish exploration. —Cnn.com Wire Service, Mercury News, 8 May 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