Frontier (noun)

1. A boundary or border separating two countries or territories.
2. A region at the edge of a settled area, typically one that is wild, remote, or undeveloped.
3. A field or domain of knowledge or activity that is in the process of being explored and developed.

Origin:

From middle english, from old french frontiere, from vulgar latin *frontaria, from latin frons ("forehead") + -aria (feminine suffix).

Examples:

1. The frontier between the two countries was heavily guarded.
2. The settlers pushed the frontier westward.
3. The explorer reached the frontier of the unknown jungle.
4. The scientist was working on the frontier of physics.
5. The company was pushing the frontier of technology.