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.