perimeter perimeter /pɝˈɪmətɝ/

noun

1. The size of something as given by the distance around it  ( perimeter , circumference )
   the size of something as given by the distance around it
   
   
2. A line enclosing a plane areas  ( perimeter )
   a line enclosing a plane areas
   
3. The boundary line or the area immediately inside the boundary  ( perimeter , border , margin )
   the boundary line or the area immediately inside the boundary
     The river formed a natural border between the two countries, separating them geographically.
   The river formed a natural border between the two countries, separating them geographically.