adversary party; opposing party
Formal legal term used in litigation, arbitration, or contractual contexts to refer to the other side.
The opposing party sought termination of this contract.
We hear the adversary party's argument in court.
Compound of 相手方 (あいてがた, 'the other side') and 当事者 (とうじしゃ, 'party concerned'). A formal legal term.