antichthon
A hypothetical Earth on the opposite side of the Sun.
obsolete
It is curious at least, that his Philosophy obliged Pythagoras, to make the Solar System #61;10, tho' his imperfect Astrognosy reduced him to the shift of including the moon, and imagining an Antiχθων ["anti-earth, anti-world"] and taking the whole as a completory Unit—Sun, Mer[cury,] Ven[us,] Earth, Moon, Mars, Jupiter, Saturn, Antichthon #61;9#43; Solar Syst[em] #61;1#61;10
Where it comes from
From Ancient Greek ἀντίχθων (antíkhthōn), from ἀντί (antí, “against, opposite”) + χθών (khthṓn, “Earth”).
Adapted from the Wiktionary entry antichthon (CC BY-SA 4.0).