Earth's equator. Credit: NASA.
In general, the equator is the great circle on the surface of a body formed by the intersection of the surface with the plane passing through the center of the body perpendicular to the axis of rotation.
The terrestrial equator is an imaginary line drawn on the Earth's surface such that all points on it are equidistant from the north and south poles, i.e., it is the line circling the Earth at 0 degrees latitude, dividing the north and south hemispheres.
The celestial equator is lies directly above the Earth's equator and is used as a reference to determine the position of a star or other celestial object using an astronomical coordinate system such as that based on equatorial coordinates.