outer
adjective /ˈaʊtə(r)/
  /ˈaʊtər/
[only before noun]- on the outside of something synonym external
- the outer layers of the skin
 - While the chassis is made of steel, the outer shell is plastic.
 - The attackers began to enter through the outer walls.
 - The stair winds up between the inner and outer walls of the house.
 
 - furthest from the inside or centre of something
- I walked along the outer edge of the track.
 - the outer suburbs of the city
 - Outer London/Mongolia
 - (figurative) to explore the outer (= most extreme) limits of human experience
 
 opposite inner 
Word Originlate Middle English: from out + -er, replacing earlier utter in its obsolete sense ‘outer’.