The word "polygon" comes from Greek.  "Poly" means "many," and "gon" has the meaning of "angle."  So a polygon is a figure with "many angles."  Another way to think of polygon is a figure with many sides, because angles have sides.  But this is not clear enough to distinguish what a polygon really is.  Some polygons you already know include triangles and rectangles.  In geometry we use definitions to classify objects that have certain properties in common and then state theorems about those properties.  For polygons those properties will relate to angles.

Recall that a good definition has to satisfy four conditions:

        1)    It names the term being defined.

        2)    It identifies the set to which the term belongs.

        3)    It states the properties that distinguish the term from others in the set.

        4)    It is reversible.

To properly define a polygon, we need to first understand what kinds of objects belong to the set of all polygons.  The main concerns in geometry are developing theorems that apply to all these objects.  In the next lesson we will state some theorems about the relationships between angles and sides of polygons.  But first let us consider some examples.  One condition is that polygons are 2-dimensional figures.  The question then is, which of those figures can properly be classified as polygons?