Perimeter/pəˈrɪmɪtər/ Definition 1 of 2Pro Tip 1/3Geometry Context Perimeter is a basic term in geometry often used in mathematics education. Find the perimeter of this triangle. Pro Tip 2/3Linear Measure Perimeter refers to a one-dimensional measure, even if the shape is two-dimensional. The perimeter of our garden is longer than yours. Pro Tip 3/3Addition of Sides It's the sum of all side lengths of a polygon, not just a measure of one side. To get the perimeter, add all the sides of the quadrilateral.