Lewis & Short

adversārĭus, a, um, adj. [adversus].

  1. I. Turned toward one or lying before one’s eyes; hence, adversārĭa, ōrum (sc. scripta), in mercantile language, a book at hand in which all matters are entered temporarily as they occur, a waste-book, day-book, journal, memoranda, etc.: Quid est quod neglegenter scribamus adversaria? quid est. quod diligenter conficiamus tabulas? Qua de causa? Quia haec sunt menstrua, illae sunt aeternae: haec delentur statim, illae servantur sancte, etc., Cic. Rosc. Com. 2, 5 and 7.
  2. II. Standing opposite or opposed to one, as an antagonist, in any kind of contest, in which the contending parties may be the best friends, e. g. in elections, auctions, discussions, etc. (cf. Doed. Syn. 4, 395; in gen., only of persons, while contrarius is used of things, Front. Differ. 2198 P.).
    1. A. Adj.: tribunus seditiosis adversarius, Cic. Clu. 34, 94: vis juri adversaria, id. Caecin. 2: opinio oratori, id. de Or. 2, 37: duces, id. Phil. 3, 8: populus, adversarius, invidus etiam potentiae, in hostile opposition to those in power, Nep. Timoth. 3: factio, id. Phoc. 3: frater, Hor. Ep. 1, 18, 63 al.
    2. B. Subst.: adversārĭus, i, m., an antagonist, opponent, adversary, an enemy, rival (the most usual class. signif. of the word): valentiorem nactus adversarium, Plaut. Capt. prol. 64: injuria adversariūm, Ter. Hec. prol. alt. 14; cf. id. Ad. prol. 2: tribuni plebis illius adversarii, defensores mei, Cic. Mil. 15; so id. Quint. 2; id. Vatin. 1; id. Har. Resp. 16, 24; Nep. Dion. 7; Hor. S. 1, 9, 75.
      Of wrestlers and other athletæ: pugiles etiam cum feriunt adversarium … ingemiscunt, Cic. Tusc. 2, 23, 56; also, in auctions, of opposing bidders: res major est quam facultates nostrae praesertim adversario et cupido et locuplete, Cic. Att. 12, 43; cf. id. ib. 13, 31.
      In Cic. also in the fem.: adversārĭa, ae: est tibi gravis adversaria constituta et parata, incredibilis quaedam exspectatio, id. Fam. 2, 4, 2; and in the neutr. plur.: adversārĭa, ōrum, the arguments, assertions of the antagonist, Cic. Or. 35, 122.
      Note: The histt. more freq. than Cic. and Hor. use adversarius like hostis for an enemy in war: adversarios in fuga esse, Nep. Them. 4: multitudo adversariorum, id. Dat. 6: montem occupat, ne forte cedentibus adversariis receptui foret, Sall. J. 50; Suet. Caes. 30, 36, 68; id. Dom. 1; Curt. 3, 11; Vulg. Deut. 20, 4; Aur. Vict. Vir. Illustr. 75, 8; 69, 2; cf. advosem in Fest. p. 25 Müll.