1. (a.) Tending to counteract.

2. (n.) One who, or that which, counteracts.

adversary adversative adversive alien antagonistic anti antipathetic antithetic antonymous at cross-purposes balancing clashing compensating competitive con conflicting confronting contradictory contradistinct contrapositive contrarious contrary contrasted converse counter counteractive counterbalancing counterpoised countervailing cross dead against deleterious detrimental difficult disaccordant disadvantageous discordant discrepant dissentient enemy eyeball to eyeball fractious hard harmful hostile hurtful impeding in opposition inconsistent inimical injurious inverse miserable negative noncooperative not easy obstinate obstructive obverse opponent opposed opposing opposite oppositional oppositive oppugnant overthwart perverse prejudicial recalcitrant refractory repugnant reverse rigorous rival sinister squared off stressful troublesome troublous trying uncooperative unfavorable unfriendly unpropitious unsatisfactory untoward wretched