Definition:Right Angle/Perpendicular/Plane

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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 $EF$.