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 next frontier is taking responsibility for what happens after. — Catherine Salfino, Footwear News, 24 Aug. 2026

My Antonia The last installment in Willa Cather’s Prairie Trilogy,My Antonia immortalizes the American Midwest and the lives of neighbors settling on the frontier. — Caroline Rogers, Southern Living, 22 Aug. 2026

While memory infrastructure startups like Mem0 are emerging, frontier labs are increasingly offering native memory solutions. — Josipa Majic Predin, Forbes.com, 22 Aug. 2026

In 2015, Turkey had shot down a Russian jet over its Syrian frontier. — Andrew Wilks, Los Angeles Times, 22 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