Main Entry: fron·tier

Pronunciation: \ˌfrən-ˈtir, ˈfrən-ˌ, frän-ˈ, ˈfrän-ˌ\

Function: noun

Etymology: Middle English fronter, from Anglo-French frountere, fronter, from front

Date: 15th century

1 a : a border between two countries b obsolete : a stronghold on a frontier
2 a : a region that forms the margin of settled or developed territory b : the farthermost limits of knowledge or achievement in a particular subject 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

— frontier adjective