Perimeter is the measure of the distance around a closed two-dimensional object.  An example of when perimeter is used is to measure the distance a fence around an area would require. 

For polygons, we find perimeter by adding the lengths of each segment in the polygon.  For circles, we call the perimeter the circle's circumference, and use formula C = 2*pi*r, where r is the radius of the circle.

Further perimeter formulas are shown on this formula sheet.  Look for "p=..." to find perimeter formulas.