Definition: Any
polygon
that is not a
regular polygon.
A polygon whose sides are not all the same length or whose
interior angles
do not all have the same measure.

Try this
Adjust the polygon below by dragging any orange dot. Note that only the vertex you drag is affected.

An irregular polygon is any
polygon
that is not a
regular polygon.
It can have sides of any length and each
interior angle
can be any measure.
They can be
convex
or concave, but all concave polygons are irregular
since the interior angles cannot all be the same. If you drew a polygon at random, it would probably be irregular.

Specific irregular polygons
such as a
parallelogram
have some interesting properties and have their own web pages.

Equilateral Polygons

Equilateral means that all sides are the same length. But as can be seen from the figure below, this can still be irregular.
This figure has all sides the same length, but it does not have all angles the same, so it is irregular.

Attributes

Vertex

The vertex (plural: vertices)
is a corner of the polygon. In any polygon, the number of sides and vertices are always equal.

Side

The sides are the straight
line segments that make up the polygon.

Area

There is no easy formula for the area of an irregular polygon,
but see Area of a Irregular Polygon for some general approaches to the problem.