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

Founders must simultaneously stay close to the customer, their purpose, and the frontier. — Michael Wee, Forbes.com, 3 Aug. 2026

Then on Thursday, apparently not wanting to be left out of the publicity party, rival AI frontier lab Anthropic confessed its own sandbox breakout incidents, three at the present accounting. — Clarence Page, Chicago Tribune, 3 Aug. 2026

That includes how the administration will define frontier AI models and whether open weight AI models – or AI models that can be downloaded to one’s computer and customized – will be part of it. — Hadas Gold, CNN Money, 3 Aug. 2026

District 8 is also home to some of the top growth frontiers as the suburbs give way to farmland that’s a source of new land for developers. — Douglas Hanks, Miami Herald, 3 Aug. 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