an imaginary line around the Earth parallel to the equator
an imaginary great circle on the surface of the earth passing through the north and south poles at right angles to the equator
(n) half of the terrestrial globe or celestial sphere, esp. one of the halves into which the earth is divided.
meridian at zero degree longitude from which east and west are reckoned (usually the Greenwich longitude in England)
an imaginary line around the Earth forming the great circle that is equidistant from the north and south poles