The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the Earth's equator. This reference plane bases the equatorial coordinate system. In other words, the celestial equator is an abstract projection of the terrestrial equator into outer space. As a result of the Earth's axial tilt, the celestial equator is currently inclined by about 23.44° with respect to the ecliptic plane.
An observer standing on the Earth's equator visualizes the celestial equator as a semicircle passing directly overhead through the zenith. As the observer moves north (or south), the celestial equator tilts towards the opposite horizon. The celestial equator is defined to be infinitely distant (since it is on the celestial sphere); thus, the observer always sees the ends of the semicircle disappear over the horizon exactly due east and due west, regardless of the observer's position on Earth. (At the poles, though, the celestial equator would be parallel to the horizon.) At all latitudes the celestial equator appears perfectly straight because the observer is only finitely far from the plane of the celestial equator but infinitely far from the celestial equator itself.
Celestial bodies other than Earth also have similarly defined celestial equators.