Definition of: frontier

(frun·tir′) noun
1. The part of a nation's territory that abuts upon another country; the border; confines.
2. That portion of a country bordering on the wilderness, newly or thinly settled by pioneer settlers.
3. Any region of thought or knowledge not yet explored: a frontier of science.
4. Obs. A fort. See synonyms under BOUNDARY.
—adjective Of, from, inhabiting, situated on, or characteristic of a frontier. [<OF frontiere <front FRONT]