Polygon
From Greek: polys "many" + gonia "angle"
Definition: A number of coplanar line segments, each connected end to end to form a closed shape.
Try this Drag the orange dots on each vertex to reshape the polygon. It is initially an irregular polygon; change it by clicking on "make regular", and change the number of sides.

Polygons are one of the most all-encompassing shapes in geometry. From the simple triangle up through squares, rectangles, trapezoids, to dodecagons and beyond.

In the figure above click on "make regular" and then gradually change the number of sides to see familiar shapes.

Types of Polygon

Regular A polygon with all sides and interior angles the same. Regular polygons are always convex.  See Regular Polygons
Irregular Each side may a different length, each angle may be a different measure. The opposite of a regular polygon. See Irregular Polygons
Convex All interior angles less than 180°,and all vertices 'point outwards' away from the interior. The opposite of concave. Regular polygons are always convex. See Convex Polygons
Concave One or more interior angles greater than 180°. Some vertices push 'inwards' towards the interior of the polygon. The opposite of convex.
See Concave Polygons
Self-intersecting
or Crossed

A polygon where one or more sides crosses back over another side, creating multiple smaller polygons. Most of the properties and theorems concerning polygons do not apply to this shape. It is best considered as several separate polygons.

A polygon that in not self-intersecting in this way is called a simple polygon.

Properties of all Polygons (regular and irregular)

Interior angles The interior angles of a polygon are those angles at each vertex on the inside of the polygon. See Polygon Interior Angles
Exterior Angles The angle on the outside of a polygon between a side and the extended adjacent side.   See Polygon Exterior Angles
Diagonals The diagonals of a polygon are lines linking any two non-adjacent vertices.   See Diagonals of a Polygon
Area For regular polygons there are various ways to calculate the area.
See Area of a Regular Polygon
For irregular polygons things are a little harder since there no general formulae.
For a general approach see Area of an Irregular Polygon

Perimeter The distance around a polygon. The sum of its side lengths.   See Perimeter of a Polygon  


Properties of Regular Polygons

Apothem (inradius) The apothem of a polygon is a line from the center to the midpoint of a side. This is also the inradius - the radius of the incircle (see below).  See Apothem of a Polygon
Radius (circumradius) The radius of a regular polygon is a line from the center to any vertex. It is also the radius of the circumcircle of the polygon (see below).  See Radius of a Polygon
Incircle The incircle is the largest circle that will fit inside a regular polygon. Its radius is the apothem of the polygon (see above).   See Incircle of a Polygon
Circumcircle The circle that passes through all the vertices of a regular polygon. Its radius is the radius of the polygon (see above).   See Circumcircle of a Polygon

Named Polygons

Many polygons have names based on the number of sides. A 5-sided polygon is called a pentagon for example. There are some that wish to name every possible polygon, but there seems little point in doing so. For example a 42-sided polygon is called a "tetracontakaidigon".

Beyond about 10 sides, most people call them an "n-gon". For example a 15-gon has 15 sides. This seems easier to remember and understand. However, there are some names that do occur in everyday experience:

Triangle 3 sides
Quadrilateral   4 sides
Tetragon 4 sides
Pentagon 5 sides
Hexagon 6 sides
Heptagon 7 sides
Octagon 8 sides
Decagon 10 sides
Dodecagon 12 sides
But if you would prefer to call a heptagon a 7-gon for example, that's fine. Everyone will know what you mean.

Related polygon topics

General

Types of polygon

Area of various polygon types

Perimeter of various polygon types

Angles associated with polygons

Named polygons