Being perpendicular is a property of lines in a plane. Generally, when the term is used, it refers to the definition of perpendicular in Euclidean geometry.
Two lines and are said to be perpendicular if they intersect in right angles. We denote this relationship by . In the Cartesian coordinate system, a line with slope is perpendicular to every line with slope and no others.
For non-linear objects
One can also discuss perpendicularity of other objects. If a line intersects a plane at a point , we say that if and only if for every line in passing through , .
If a plane intersects another plane in a line , we say that if and only if: for line and passing through , and implies .
Two linear graphs in the Cartesian coordinate plane are perpendicular if and only if one's slope is the negative reciprocal of the other's. This means that their slopes must have a product of .