polar coordinates

1. In the plane, a system of curvilinear coordinates in which a point is located by its distance r from the origin (or pole) and by the angle θ which a line joining the given point and the origin makes with a fixed reference line, called the polar axis.

The relations between rectangular Cartesian coordinates and polar coordinates are

where the origins of the two systems coincide and the polar axis coincides with the x axis.

1. In space,
   same as spherical coordinates.
   
   See also cylindrical coordinates.