'counter-Earth' originates from modern English, formed as a compound of the prefix 'counter-' (ultimately from Latin 'contra', meaning 'against' or 'opposite') and 'Earth' (from Old English 'eorþe', meaning 'ground, soil, land').

'counter-Earth' is effectively an English calque of the ancient Greek term 'Antichthon' (ἀντίχθων), used by Pythagoreans to denote an 'opposite earth'; Greek 'anti-' meant 'opposite' and 'chthon' meant 'earth' or 'land'. The modern English compound arose later to express the same idea.

Originally it referred specifically to the Pythagorean hypothetical planet 'Antichthon'; over time the term broadened to denote any hypothetical or fictional planet occupying Earth's position on the opposite side of the sun or a figurative 'mirror' Earth in fiction.