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.
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 .