other side; other party; other end (e.g. of phone connection); opposition
相手側の言い分を聞いた。
I heard what the other party had to say.
交渉では相手側の条件を十分に検討した。
In the negotiations, we thoroughly examined the other party's terms.
裁判で相手側の弁護士が予想外の証拠を提出してきた。
In court, the opposing counsel submitted unexpected evidence.