A quadrilateral is a closed two-dimensional figure that has four sides, four angles, and four vertices. It is a polygon with four edges and four corners. The word "quadrilateral" is derived from the Latin words "quadri," meaning four, and "latus," meaning side. Quadrilaterals can be formed by joining four non-collinear points. The sum of the interior angles of a quadrilateral is always 360 degrees. There are different types of quadrilaterals, including:

• Square: A quadrilateral with all four sides of equal length and all four angles measuring 90 degrees.
• Rectangle: A quadrilateral with all four angles measuring 90 degrees, but opposite sides are of equal length.
• Rhombus: A quadrilateral with all four sides of equal length, but opposite angles are not necessarily equal.
• Trapezoid: A quadrilateral with at least one pair of parallel sides.

Quadrilaterals can also be classified as convex or concave. A convex quadrilateral has all interior angles measuring less than 180 degrees and both diagonals lie inside the quadrilateral. A concave quadrilateral has at least one interior angle measuring greater than 180 degrees and one diagonal lies outside the quadrilateral.