What Is Perimeter, in Geometry?

Perimeter is the length of a shape's outline

A shape's perimeter is its outline. When we talk about perimeter in geometry, we usually mean the length of the perimeter—which we can find by just adding up the lengths of all its sides.

In the image below, the shape has six sides. The sides are of lengths (going counterclockwise from the upper right corner): $5, 8, 9, 5, 12,$ and $15$. To find the perimeter we add: $$5+8+9+5+12+15 = 54.$$