Perpendicular
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.
Definition
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 .
Coordinate Plane
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 
.
