Definition:Right Angle

Definition
Right angles are angles that are equal to half of a straight angle.

It can be shown that the measurement of a right angle is $90^\circ$ or $\dfrac \pi 2$ radians (see here).

Perpendicular
As Euclid defined it:

"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 $AB$.