frontier frontier /frənˈtɪr/

noun

1. () A wilderness at the edge of a settled area of a country  ( frontier )
   a wilderness at the edge of a settled area of a country
    The individualism of the frontier in Andrew Jackson's day
   the individualism of the frontier in Andrew Jackson's day
   
2. An undeveloped field of study; a topic inviting research and development  ( frontier )
   an undeveloped field of study; a topic inviting research and development
    He worked at the frontier of brain science
   he worked at the frontier of brain science
   
3. An international boundary or the area (often fortified) immediately inside the boundary  ( frontier )
   an international boundary or the area (often fortified) immediately inside the boundary