Counter-Earth is a non-existent Earth-like hypothetical planet on Earth's orbit but on the other side of the Sun. Hypothesized by the Pythagorean school; used by John Norman as the setting for his Gor novels.
A similar concept was used in Gerry Anderson's movie Journey to the Far Side of the Sun, aka Doppelganger, in which the Counter-Earth is identical to Earth in every respect except that left and right are reversed. (But if that were so, then surely the planet would also orbit the sun in the opposite direction and collide with the Earth....)
If such a planet actually existed, it would be permanently hidden behind the sun, but would nevertheless be detectable from Earth because of its gravitational influence upon the other planets of the Solar System. No such influence has been detected, and indeed the space probes sent to Venus, Mars, etc, could not have successfully flown by or landed on their targets if a Counter-Earth existed, but was not accounted for in navigational calculation.
