what is perimeter

The perimeter is the distance around a region, measured in suitable units of length

The perimeter of a circle has its own special name, the circumference

here is is and abundance of examples of perimeter
https://www.google.ca/search?q=perimeter&es_sm=119&tbm=isch&tbo=u&source=univ&sa=X&ved=0CE8QsARqFQoTCIeQuoL7-McCFUcUkgodBloEFg&biw=1605&bih=858