The other side or party in a negotiation, phone call, match, etc.; the position or party opposite you.
電話が切れたので、相手側にもう一度かけ直してください。
The call was dropped, so please call the other party back.
契約の条件について相手側と最終確認を行った。
We made a final confirmation with the other party about the contract terms.
今回の交渉では相手側の事情も考慮する必要がある。
In these negotiations we need to take the other side's circumstances into account.