9 perimeter

perimeter
n
1 (length of the) outer edge of a closed geometric shape 周; 周边; 边缘; 周长.
2 boundary of an area 周围的界限
    Guards patrolled the perimeter of the airfield. 卫兵沿机场四周巡逻.
    [attrib 作定语]
the perimeter fence 周围的篱笆. Cf 参看 circumference.