Equator

noun

1. (often “the Equator”) An imaginary great circle around the Earth, equidistant from the two poles, and dividing earth's surface into the northern and southern hemisphere.
2. A similar great circle on any sphere, especially on a celestial body, or on other reasonably symmetrical three-dimensional body.
3. The midline of any generally spherical object, such as a fruit or vegetable, that has identifiable poles.

   "Slice the onion through the equator."

4. The celestial equator.