A 4-sided regular polygon with all sides equal and all internal angles 90°

Try this Drag the orange dots on each vertex
to reshape the square.

The square is probably the best known of the quadrilaterals.
It is defined as having all sides equal, and its interior angles all right angles (90°).
From this it follows that the opposite sides are also parallel.

A square is simply a specific case of a regular polygon, in this case with 4 sides.
All the facts and properties described for regular polygons apply to a square.
See Regular Polygons

Attributes

Vertex

The vertex (plural: vertices)
is a corner of the square. Every square has four vertices.

Perimeter

The distance around the square. All four sides are by definition the same length, so the perimeter is four times the
length of one side, or:

Each diagonal of a square is the
perpendicular bisector
of the other.
That is, each cuts the other into two equal parts, and they cross and right angles (90°).
The length of each diagonal is