(1):

(n.) The great circle of the celestial sphere, coincident with the plane of the earth's equator; - so called because when the sun is in it, the days and nights are of equal length; hence called also the equinoctial, and on maps, globes, etc., the equinoctial line.

(2):

(n.) The imaginary great circle on the earth's surface, everywhere equally distant from the two poles, and dividing the earth's surface into two hemispheres.