adversary

: one that contends with or opposes another; especially : any of the opposing parties in a legal action : any of the opposing parties in a legal action

: of, relating to, or involving opposing parties or interests; specifically : of, relating to, or involving a system of justice in which opposing parties usually represented by counsel present evidence to an impartial decision-maker (as a jury) by a process of questioning witnesses under the supervision of a judge — compare accusatorial, inquisitorial : of, relating to, or involving a system of justice in which opposing parties usually represented by counsel present evidence to an impartial decision-maker (as a jury) by a process of questioning witnesses under the supervision of a judge — compare accusatorial, inquisitorial