In a polar coordinate system, a fixed point
O is called the pole or origin. The polar axis is
usually a horizontal ray directed toward the right from the pole.
The location of point P in the polar coordinate system can be
identified by polar coordinates in the form (r, 
).
If a ray is drawn from the pole through point P, the distance
from the pole to point P is | r |. The
measure of the angle formed by ray OP and the polar axis is . The angle can be measured in degrees or
radians.
If r is positive, 
is
the measure of any angle in standard position that has ray OP
as its terminal side.
If r is negative, is
the measure of any angle that has the ray opposite ray OP as its
terminal side.
An equation that uses polar coordinates is
called a polar equation. For example, 
is a polar equation. A polar graph represents the solution
set which is the set of points whose coordinates (r, )
satisfy a given polar equation.