frontier

noun

1. the border between two countries
2. ☆
   1. that part of a settled, civilized country which lies next to an unexplored or undeveloped region
   2. the developing, often still uncivilized or lawless, region of a country
3. any new field of learning, thought, etc. or any part of a field that is still incompletely investigated: often used in pl.: the frontiers of medicine

adjective