A quadrilateral is a geometric figure formed by joining four points, any three of which are non-collinear, in some order. In other words, a quadrilateral is any closed, four-sided figure. Two examples are shown below:

Asymmetric Quadrilateral

Asymmetric Quadrilateral

We note that a quadrilateral has four sides, four angles, and four vertices. When we have to specify a quadrilateral, we do it keeping in mind the order of the vertices. For example, consider the following quadrilateral:

Quadrilateral with unequal sides

We could name this quadrilateral ABCD, BCDA, ADCB, DCBA, etc, but we cannot name it ACBD, BACD, etc., since in the latter names, the order of the vertices is wrong.

A quadrilateral ABCD will have two diagonals: AC and BD.

Quadrilateral with diagonals