:a border between two countries
- the frontier between Canada and the U.S.
:a stronghold on a frontier
:a region that forms the margin of settled or developed territory
- were sent on an expedition to explore the western frontier
:the farthermost limits of knowledge or achievement in a particular subject
:a line of division between different or opposed things
- the frontiers separating science and the humanities
- —R. W. Clark
: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