system of locating points in a plane with reference to a fixed point (origin) and an axis through that point. The coordinates are written (r,θ), in which r is the distance from the origin to any desired point and θ is the angle made by the vector r and the axis. A simple relationship exists between Cartesian coordinates given in terms of two reference axes (x,y) and the polar coordinates (r,θ), namely: x = r cos θ, and y = r sin θ.

Polar coordinates, like Cartesian coordinates, may also be used to locate points in three-dimensional space. The system used involves again the radius vector r, which gives distance from the origin, the angle θ, measured between r and the z axis, and a second angle ϕ, measured between the x axis and the projection of r in the x,y plane. This system is essentially identical to that of spherical coordinates; points on Earth, for example, are located in terms of latitude and longitude, which express angles measured with respect to the axis of the Earth's rotation and with respect to an arbitrary reference of longitude (the Greenwich meridian).