Circle of latitude

A circle of latitude or parallel is an imaginary east-west line that connects all points on Earth that lie on the same latitude. The Equator is a circle of latitude, as are the polar circles and the parallels that define the Tropic of Cancer and the Tropic of Capricorn.

The Earth's equator is the only circle of latitude that is also a great circle; all other circles of latitude are small circles. Circles of latitude are also called parallels, as they are parallel to each other. The position of a point along a circle of latitude is defined by its longitude. Circles of latitude lie at right angles to meridians, or the north-south great circles that mark longitude. While meridians converge towards the poles, circles of latitude remain equidistant.

Important named circles of latitude
Besides the Equator, four other circles of latitude are named because of the role they play in  the geometrical relationship with the Earth and the Sun:
 * Arctic Circle: 66° 33′ 39″ N
 * Tropic of Cancer: 23° 26′ 21″ N
 * Tropic of Capricorn: 23° 26′ 21″ S
 * Antarctic Circle: 66° 33′ 39″ S