An Etymological Dictionary of Astronomy and Astrophysics

A person or thing that corresponds to or has the same function as another person or thing in a different place or situation (OxfordDictionaries.com).
→ electromagnetic counterpart, → optical counterpart, → radio counterpart.

Etymology (EN): M.E., from O.Fr. contrepartie, from contre “facing, opposite,” → counter-, + partie “copy of a person or thing,” originally feminine p.p. of partir “to divide.”

Etymology (PE): Hamtâ “counterpart, resembling, equal,” from ham- “together, with; same, equally, even,” → com-,

• tâ “fold, plait, ply; piece, part,” also a multiplicative suffix; Mid.Pers. tâg “piece, part.”