Area

The total number of square units that fill a given shape.
Circumference

The distance around the edge of a circle; the "perimeter" of a circle.
Diameter

A line segment from one point on the edge of the circle to another point on the edge, passing through the center of the circle. The longest line segment that cuts across the circle from one point to another.
Perimeter

The distance around the edge of a shape; in general, the sum of the lengths of its sides.
pi,
Π

The ratio of the circumference of a circle to its diameter.
Π
is roughly equal to 3.14.
Polygon

A closed shape made up of several straight line segments.
Radius

A line segment from the center of the circle to a point on the edge of the circle. Half of a diameter.
Rectangle

A 4sided polygon in which opposite sides have the same length and all angles have the same measure (90 degrees).
Square

A 4sided polygon in which all sides are equal and all angles have the same measure (90 degrees).