Definition:Quadrilateral

A quadrilateral is a polygon with four sides.

Because it is a polygon, it follows that it also has four vertices.

Square
A square is a regular quadrilateral.

That is, the angles and sides of a square are all right angles:



As Euclid put it: "A square is [a quadrilateral figure] which is both equilateral and right-angled."

Oblong
An oblong is a quadrilateral whose angles are all right angles, but whose sides are not all the same length:



As Euclid put it: "An oblong [is a quadrilateral figure] which is right-angled but not equilateral."

Rectangle
A rectangle is a quadrilateral all of whose angles are equal to a right angle, and whose sides may or may not all be the same length.

That is, both squares and oblongs are both rectangles.

The word "oblong" is rarely seen nowadays; "rectangle" is the term usually used instead.