A polygon is a flat, 2D shape.
It has straight sides and is fully closed.
All of the sides are straight and joined up.
Polygons can have any number of sides.
A regular polygon has all sides of the same length and all sides at the same angle.
An equilateral triangle is a regular polygon; all of its sides and angles are the same.
An irregular polygon is a shape with sides and/or angles at differing lengths and sizes.
A scalene triangle has differing angles and sides.