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. Some key properties of quadrilaterals include:

• Sides: A quadrilateral has four sides or edges, which are denoted as AB, BC, CD, and DA.

• Angles: A quadrilateral has four angles, which are present at the four vertices or corners of the quadrilateral. They are denoted as ∠A, ∠B, ∠C, and ∠D.

• Vertices: A quadrilateral has four vertices, which are the points where the sides of the quadrilateral meet.

• Diagonals: A quadrilateral has two diagonals, which are line segments that connect opposite vertices of the quadrilateral. They are denoted as AC and BD.

There are many different types of quadrilaterals, including:

• Convex Quadrilaterals: In a convex quadrilateral, all interior angles are less than 180°, and the two diagonals both lie inside the quadrilateral.

• Concave Quadrilaterals: In a concave quadrilateral, one interior angle is greater than 180°, and at least one diagonal is not a part of the same region of the quadrilateral.

• Irregular Quadrilaterals: An irregular quadrilateral is a quadrilateral that does not have all sides or angles equal.

• Regular Quadrilaterals: A regular quadrilateral is a quadrilateral that has all sides and angles equal. The only regular quadrilateral is a square.

Other types of quadrilaterals include rectangles, rhombuses, trapezoids, and kites.