A quadrilateral is a four-sided shape, such as a rectangle or parallelogram. In order for a shape to be considered a quadrilateral, it has to have four straight sides and be two-dimensional.
If all of the interior angles of a quadrilateral are added together, they are equal to 360 degrees. Some quadrilaterals require specific qualifications, such as all sides have to be equal in length or the sides have to be parallel. For instance, a rhombus has two sets of parallel sides that are equal in length.
Quadrilaterals can be complex. Complex quadrilaterals, or intersecting quadrilaterals, cross over at some point. A bow tie shape is a good example of a complex quadrilateral.
Quadrilaterals can be inclusive. This means that a shape can fit more than one definition. For instance, a square can also be considered a rectangle. A rectangle has to be four-sided, and every angle must equal 90 degrees. The square also fits this definition. However, not all rectangles are squares. A square must have four sides equal in length.
Quadrilaterals are also known as quadrangles and tetragons. The term “quadrangle” references the four angles of quadrilaterals. "Tetragon" refers to the fact that the quadrilateral is a four-sided polygon.