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

The symbolic unity pose was to declare the formal launch of the New Delhi Frontier AI Commitments, a set of voluntary principles adopted by leading AI companies at the summit to advance inclusive, responsible development of frontier AI models. —Aditya Soni, USA Today, 19 Feb. 2026

To him, the desert’s vast, daunting landscape remains the ultimate frontier for paleontologists brave enough to pursue the unknown. —Mrigakshi Dixit, Interesting Engineering, 19 Feb. 2026

Rather than pouring resources into speculative frontier models, Beijing has a pressing incentive to use the technology as a more immediate productivity engine. —IEEE Spectrum, 19 Feb. 2026

Boatwright would be wise to heed this history and either take Chipotle into new frontiers or prepare for its inevitable irrelevance. —Gustavo Arellano, Los Angeles Times, 19 Feb. 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