Definition:Right Angle

Definition

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

Measurement of Right Angle

The measurement of a right angle is $\dfrac {180 \degrees} 2 = 90 \degrees$ or $\dfrac \pi 2$.

Perpendicular

In the words of Euclid:

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.

(The Elements: Book $\text{I}$: Definition $10$)

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

Sources

• 1947: William H. McCrea: Analytical Geometry of Three Dimensions (2nd ed.) ... (previous) ... (next): Chapter $\text {I}$: Coordinate System: Directions: $\S 1$. Introductory: Metrical geometry
• 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): right angle