An Etymological Dictionary of Astronomy and Astrophysics

1. A person, group, or force that opposes or attacks; opponent; enemy; foe.
   

2. A person, group, etc., that is an opponent in a contest; contestant (Dictionary.com).

Etymology (EN): M.E. adversaire; O.Fr. adversaire “opponent, adversary, rival,” from L. adversarius “opposite, hostile, contrary,” from adversus “turned against,” from → ad- + verse, → universe.

Etymology (PE): Hamestâr, from Mid.Pers. hamestâr “adversary,” from Av. hamaēstar-, from ha-, → com-, + maēd- “to bring down, to suppress.”