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?

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

1. AB, BC, CD and AD are called the sides of a quadrilateral.
2. The four corners A, B, C and D at which any two line segments intersect are called Vertices of the quadrilateral ABCD.
3. At the four verties A, B, C and D, the angles that are formed are called Interior angles of the quadrilateral.
4. Any two sides that have a common vertex are called adjacent sides.
5. Vertices next to each other are termed consecutive vertices such as A and B, B and C, C and D and D and A.
6. Vertices which are not consecutive are called non-consecutive vertices.
7. A diagonal is a line segment that joins any two non consecutive vertices of a quadrilateral .
8. In the above quadrilateral AC and BD are diagonals.
9. 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.
10.  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.