(n.) An opposing force.

accommodate adjust amend annul antipode antipodes antipole antithesis antonym atone for balance ballast be at cross-purposes be heavy be opposed to bring to nothing buffer cancel cancel out carry weight clash come to nothing compensate conflict conflict with consideration contra contradict contrapose contrast with contravene controvert converse coordinate correct counter counteract counterblast countercheck countercurrent counterforce counterinfluence counterpoint counterpoise counterpole counterpose counterpressure counterterm countervail counterweigh counterweight counterwork crosscurrent equalize equate equipoise equiponderate equivalent even even up firm firm up fit foil foul wind freeze frustrate give-and-take have weight head wind heft hold hold the scales immobilize integrate invalidate inverse juxtapose in opposition keep level lie heavy make up makeweight nail down negate negativate negative neutralize nullify obverse offset oppose opposite opposite number oppugn outweigh pin down play at cross-purposes poise proportion quid pro quo rectify redeem retain reverse run counter to set off setoff something of value square square up stabilitate stabilize steady stick strike a balance stultify the contrary the other side thwart tip the scales tit for tat transfix undercurrent undo vis-a-vis vitiate void weigh weigh heavy weigh in weigh out weight