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Perpendicular is a geometric term that may be used as a noun or adjective. The fundamental meanings pertain to the position of straight lines relative to one another, in which two lines are positioned at an angle of ninety degrees, which is defined as a right angle. Any two lines in Euclidean space which are perpendicular to one another and which intersect automatically define a plane.

Naturally, if a line is given, then a perpendicular is any line at a ninety-degree angle to that line. This is an important property in geometry and trigonometry since important properties accrue to line systems containing right angles. When graphinging, the convention is to use either an X and Y axis, or to use an X, Y, and Z axis, which are defined as being mutually perpendicular. Right triangles, too, include two perpendicular lines and so have special properties, which are the foundation of trigonometry.