A parallel (of latitude) is a line of constant latitude (circle of latitude) on a mathematical earth model.

The shape of parallels on a sphere or ellipsoid are circles on planes parallel to the equator plan and vertical to the earth axis. The radius of the circle of latitude decreases depending of the latitude. The parallel with latitude = 0° is the equator.

A set of meridians and parallels with constant coordinate offsets defines a graticule.