Equator
The equator is an imaginary great circle drawn around the Earth, exactly halfway between the North Pole and the South Pole. It divides the Earth into the Northern Hemisphere and the Southern Hemisphere and has a latitude of zero degrees ($0^\circ$).
Example
When calculating distances along the Earth's surface in three-dimensional geometry, the equator is the longest line of latitude. Its total length is approximately $2 \pi R$, where $R \approx 6370 \text{ km}$ is the radius of the Earth.