Square

The sqaure, or regular tetragon is the 4-sided regular polygon. It has 4 equal sides and 4 equal angles.

The square is one of three regular polygons that can tile the plane, the others being the equilateral triangle and regular hexagon. This tiling is called the square tiling.

Vertex coordinates
The vertices of a square of edge length 1 centered at the origin are
 * (±1/2, ±1/2).

Other kinds of quadrilaterals
Besides the regular square, there are various other kinds of quadrilaterals:

Rectangle
A rectangle has two pairs of opposite parallel sides, with all angles still being equal at 90º.

Bowtie
A bowtie, also known as a crossed rectangle, is a type of crossed quadrilateral where the vertices form a rectangle. It has one pair of opposite parallel sides.

Trapezoid
A trapezoid is a quadrilateral with one pair of parallel sides. The two vertices on either end of one of the parallel sides are identical.

Butterfly
A butterfly, also known as an antiparallelogram, is a type of crossed quadrilateral where the vertices form a trapezoid. Angles and edges come in two types.

Crossed trapezoid
A crossed trapezoid is a type of crossed quadrilateral where the vertices form a trapezoid. Angles come in two types, but unlike the butterfly, edges come in three types.