The equator is an imaginary line that circles the Earth at 0 degrees latitude, dividing the planet into the northern and southern hemispheres. It is a great circle around the Earth that is everywhere equidistant from the geographic poles and lies in a plane perpendicular to Earth’s axis. The equator is approximately 40,075 kilometers (24,901 miles) long and is the only line of latitude that is also a great circle, meaning one whose plane passes through the center of the globe. The equator is one of the five notable circles of latitude on Earth; the other four are the two polar circles (the Arctic Circle and the Antarctic Circle) and the two tropical circles (the Tropic of Cancer and the Tropic of Capricorn) .

The equator is also the intersection of the spheroid with the plane perpendicular to its axis of rotation and midway between its geographical poles. The plane of Earths equator, when projected outwards to the celestial sphere, defines the celestial equator. In astronomy, the celestial equator is the great circle in which the plane of the terrestrial equator intersects the celestial sphere; it consequently is equidistant from the celestial poles. When the Sun lies in its plane, day and night are everywhere of equal length, a twice-per-year occurrence (about March 21 and about September 23) known as equinox.

The equator receives more incoming solar radiation over the course of a year than other locations more poleward, and the Suns rays are directly overhead (that is, striking at a 90° angle with respect to the surface) at noon during both equinoxes at the equator, and they are nearly direct for the remainder of the year. Above the equator is a zone of low atmospheric pressure called the intertropical convergence zone (ITCZ), which occurs as a wide belt of converging trade winds and rising air that encircles the Earth.

The equator passes through 13 countries, including Ecuador, Colombia, Brazil, Congo, Gabon, Uganda, Kenya, and Indonesia.