Definition:Right Angle

Definition
A right angle is an angles that is equal to half of a straight angle.

The measurement of a right angle is $90^\circ$ or $\dfrac \pi 2$ radians.

Perpendicular

 *  When a straight line set up on a straight line makes the adjacent angles equal to one another, each of the equal angles is right, and the straight line standing on the other is called a perpendicular to that on which it stands. 


 * Perpendicular.png

In the above diagram, the line $CD$ has been constructed so as to be a perpendicular to the line $AB$.