Definition:Right Angle

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 $$\frac{\pi}{2}$$ rad (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."