frontier

Concise Oxford English Dictionary © 2008 Oxford University Press:
frontier /ˈfrʌntɪə, frʌnˈtɪə/

▶noun

• 1 a border separating two countries.

• 2 the extreme limit of settled land beyond which lies wilderness.

• 3 the extreme limit of understanding or achievement in a particular area.

– derivatives
frontierless adjective.

– origin ME: from OFr. frontiere, based on L. frons, front- ‘front’.