perimeter

An area around the outside of something that can be walked or measured is called the perimeter.

When we talk about the perimeter of a house or a park, we usually mean the distance around it - how far would you have to walk if you went all the way around the property? For example, a security guard might patrol the perimeter of a building at night. The word "perimeter" is often used in sports too; for instance, the perimeter of a basketball court includes both the area inside the lines where players can run and the outer boundary where fans sit.