Definition:Right Angle/Perpendicular/Plane

Definition

In the words of Euclid:

A straight line is at right angles to a plane when it makes right angles with all the straight lines which meet it and are in the plane.

(The Elements: Book $\text{XI}$: Definition $3$)

In the above diagram, the line $AB$ has been constructed so as to be a perpendicular to the plane containing the straight lines $CD$ and $EF$.

Also known as

The word normal is often used for perpendicular, particularly in the context of vector analysis.

Also, in the context of linear algebra and analysis, the word orthogonal is often encountered, which is a generalization of the concept of perpendicularity, but in a more abstract context than geometry