An Etymological Dictionary of Astronomy and Astrophysics

A coordinate system in which the position of any point (M) in a plane is specified by two coordinates: 1) ρ, which expresses the distance from a fixed point (the pole, denoted O), and 2) the number φ, which is the angle formed by
the line segment OM and a fixed reference line passing through the pole.

See also: → polar; → coordinate.