The inhabitants at two points on the globe that share a longitude and for which the sum of their degrees of latitude equals zero.
1622, Peter Heylin, Cosmographie in Four Bookes, Containing the Chorographie and Historie of the Whole World, published 1674, introduction, page 20, column 1:
Antœci are such as dwell under the same Meridian and the same Latitude or Parallel equally distant from the Æquator; the one northward, the other Southward; the days in both places being of a length; but the Summer of the one being the others winter.
1684, Thomas Burnet, The Theory of the Earth, book 2, published 1697, page 174:
Antichthones…comprehend both the Antipodes and Antœci, or all beyond the Line[.]