
Adjacent Sides
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 tensided polygon.

Diagonal
A line segment joining nonconsecutive vertices of a polygon.

Dodecagon
A twelvesided 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 sevensided polygon.

Hexagon
A sixsided 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 ninesided polygon.

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

Octagon
An eightsided polygon.

Parallelogram
A quadrilateral with two pairs of parallel sides.

Pentagon
A fivesided polygon.

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

Quadrilateral
A foursided 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 threesided polygon.

Vertex
The common endpoint of two sides of a polygon.
Terms
Formulae
Diagonal Sum  A polygon with n sides has n(n3)/2 diagonals. 
Interior Angle Sum  The interior angle sum of a polygon with n sides is 180(n2) degrees. 