### Terms

Sides of a polygon that share a vertex.

• ### Base

One of the parallel sides of a trapezoid.

• ### Center of a Regular Polygon

The point on the interior of a polygon from which all vertices of the polygon are equidistant.

• ### Closed Curve

A curve whose starting point is the same as its ending point.

• ### Concave Polygon

A polygon with one or more interior angles greater than 180 degrees.

• ### Congruent Polygons

Polygons whose sides are all congruent

• ### Consecutive Vertices

Vertices of a polygon that include the endpoints of the same side.

• ### Continuous

Without any gaps or holes.

• ### Convex Polygon

A polygon whose interior angles are all less than 180 degrees.

• ### Consecutive Angles

Angles in a polygon that share a segment as one of the sides that could be extended into a ray.

• ### Curve

An arrangement of continuous points in space.

• ### Decagon

A ten-sided polygon.

• ### Diagonal

A line segment joining nonconsecutive vertices of a polygon.

• ### Dodecagon

A twelve-sided polygon.

• ### Equiangular Polygon

A polygon whose angles are congruent.

• ### Equilateral Polygon

A polygon whose sides are congruent.

• ### Exterior Angle

The angle formed at a vertex of a polygon that lies outside of the region enclosed by a polygon.

• ### Heptagon

A seven-sided polygon.

• ### Hexagon

A six-sided polygon.

• ### Interior Angle

The angle formed, at a vertex of a polygon, that lies inside the region enclosed by the polygon.

• ### Leg

One of the nonparallel sides of a trapezoid.

• ### Median

The segment in a trapezoid whose endpoints are the midpoints of the legs of the trapezoid.

• ### Nonagon

A nine-sided polygon.

• ### Opposite sides

Sides of a quadrilateral that don't share a vertex.

• ### Octagon

An eight-sided polygon.

• ### Parallelogram

A quadrilateral with two pairs of parallel sides.

• ### Pentagon

A five-sided polygon.

• ### Polygon

A simple closed curve consisting of the union of segments that intersect each other at their endpoints.

A four-sided polygon.

• ### Rectangle

A quadrilateral with four congruent angles.

• ### Regular Polygon

An equilateral, equiangular polygon.

• ### Rhombus

A quadrilateral with four congruent sides.

• ### Side

One segment that comprises part of a polygon.

• ### Simple Closed Curve

A closed curve that does not intersect itself.

• ### Square

A quadrilateral with four congruent sides and angles.

• ### Trapezoid

A quadrilateral with one pair of parallel sides.

• ### Triangle

A three-sided polygon.

• ### Vertex

The common endpoint of two sides of a polygon.

• ### Formulae

 Diagonal Sum A polygon with n sides has n(n-3)/2 diagonals.

 Interior Angle Sum The interior angle sum of a polygon with n sides is 180(n-2) degrees.