The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference 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 planet's axial tilt, the celestial equator is currently inclined by about 23.44° with respect to the ecliptic plane (plane of the earth's orbit).
An observer standing on Earth's equator visualizes the celestial equator as a semicircle above (east-west, passing through the zenith, the point directly overhead). 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 appear/disappear on the horizon due east and due west, regardless of the observer's position on Earth. (At the poles, though, the celestial equator forms a circle parallel to the horizon (depending on the day/season of the year, above, below (and on the equinoxes on) the horizon.) At all latitudes, the celestial equator is a uniform arc or circle because the observer is only finitely far from the plane of the celestial equator, but infinitely far from the celestial equator itself.
Celestial objects near the celestial equator (such as the sun) appear above the horizon in some or all of the seasons from all places on earth — furthermore they daily or nightly culminate (reach their meridian, highest point in the sky) within 23 degrees or so of the sky's highest point, the zenith, as seen from any given point in the tropics. The celestial equator currently passes through these constellations:
These, by definition, are the most globally visible constellations.
Celestial bodies other than Earth also have similarly defined celestial equators.