The Collaborative International Dictionary of English v.0.48:

Antichthon \An*tich"thon\, n.; pl. Antichthones. [Gr. ?;
   'anti` against + ? the earth.]
   1. A hypothetical earth counter to ours, or on the opposite
      side of the sun. --Grote.
      [1913 Webster]

   2. pl. Inhabitants of opposite hemispheres. --Whewell.
      [1913 Webster]