We have seen that a triangle is a simple closed curve. A triangle has three sides. All of the three sides are line-segments.
In plane geometry there are several shapes whose sides are line segments.

Such geometric shapes made up on line segments are called polygons.

Definition

A polygon is a simple closed curve formed by the union of a finite number of line segments, no two of which in succession are collinear.

The line segments are called sides of the polygon, and the end points of the sides are called vertices of the polygon.

Example

Figure 1 above shows a polygon with five sides and five vertices.
Figure 2 is not polygon because it is not a simple closed curve.
Figure 3 is a polygon with four sides and four vertices.

Definition

Convex and Concave polygons
A ploygon is a convex ploygon if each of its interior angles mesures less than 180°. A
ploygon is a concave ploygon if it has at least one interior angle with a measure greater
than 180°

Example