Perpendicular straight lines
Straight lines forming a right angle with each other (such straight lines need not intersect in space). A straight line $l$ and a plane $\alpha$ are said to be mutually perpendicular if $l$ is perpendicular to any straight line lying in $\alpha$. See Orthogonality for a generalization of the concept of perpendicularity.
Perpendicular straight lines. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Perpendicular_straight_lines&oldid=31464