Included angle
The angle made by two consecutive sides of a polygon.

Included side
The side between two consecutive angles in a polygon.

Polygon
A polygon is a two-dimensional geometric figure with these characteristics: 
It is made of straight line segments.
Each segment touches exactly two other segments, one at each of its endpoints. 
It is closed -- it divides the plane into two distinct regions, one inside and the other outside the polygon.

Side of a polygon
A single segment from the union that forms a polygon.

Triangle
A polygon with three sides.

Triangle inequality
The triangle inequality says that for three lengths to make a triangle, the sum of the lengths of any two sides must be greater than the third length.