Terms
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.
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.
