ἱσημερινός

The "belt" of a sphere (or geometrically: the great circle) that is exactly in the middle between both poles (or: 90° away from both poles) is called "the equator".

In the image, the blue line is the celestial equator. The orange line is also a great circle but it is tilted and it's distance from the poles differes. In order to make it an equator, we would have to choose different poles.