A quadrilateral is a four sided closed plane figure

It is formed by the intersection of four line segments at four distinct points in a same plane.

Quadra means four and lateral means sides.

In the above figure 1, a quadrilateral is formed by the intersection of four line segments AB, BC, CD and DA.

A, B, C and D are four corners of the quadrilateral formed by the intersection of the four straight lines AB, BC, CD and DA.

What do you call the above figure?

A Quadrilateral ABCD.

That is how, a quadrilateral is called by naming its four corners prefixed with the word Quadrilateral, just as we would say ‘Triangle ABC’.

A quadrilateral is a four sided closed figure formed in one same plane.

No two line segments out of the four AB, BC, CD and AD are collinear, i.e. lie on a same straight line.

Note the following terms in a quadrilateral

- AB, BC, CD and AD are called the
**sides**of a quadrilateral. - The four corners A, B, C and D at which any two line segments intersect are called Vertices of the quadrilateral ABCD.
- At the four verties A, B, C and D, the angles that are formed are called Interior angles of the quadrilateral.
- Any two sides that have a common vertex are called adjacent sides.
- Vertices next to each other are termed consecutive vertices such as A and B, B and C, C and D and D and A.
- Vertices which are not consecutive are called
**non-consecutive vertices.** - A
**diagonal**is a line segment that joins any two non consecutive vertices of a quadrilateral . - In the above quadrilateral AC and BD are
**diagonals.** - The two diagonals AC and BD meet each other at point O.

O is called the**point of intersection**of the diagonals of the quadrilateral. - Any side can serve for
**Base**. The**Altitude**or**Height**will then be the line drawn perpendicular to the base from the**opposite vertex**.

In the quadrilateral ABDC below, if CD is base, then straight line AE perpendicular to CD is Height or Altitude.