term:

Adversary

adversary n

: one that contends with or opposes another
;esp
: any of the opposing parties in a legal action
adj : of, relating to, or involving opposing parties or interests
;specif
: 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