Definition
Perimeter
Perimeter tells how far one complete trip around a closed flat figure would be.
What it means
Imagine walking once around the edge of a shape and returning to where you started. The whole distance you walked is the perimeter. On a polygon, count every outside side once; do not count the space inside.
Remember it
Trace one complete lap around the edge.
Example
A triangle has side lengths 5 cm, 7 cm, and 9 cm. Its perimeter is 5 + 7 + 9 = 21 cm.
Watch for
Perimeter does not measure the inside of a shape. The inside is area and uses square units.
Common mix-up
Do not multiply side lengths to find perimeter; add the lengths that make the whole boundary.
Exact meaning
Perimeter is the length of the complete boundary of a closed two-dimensional figure. For a polygon, it is found by adding every side length once and is reported in linear units.