If someone acts unilaterally, they do something by themselves, without asking the other side or waiting for the other side to agree. The word is used when a decision touches two or more people, groups, or countries, but only one of them actually made it. A country can unilaterally leave an agreement; a company can unilaterally change the price in a contract; a government can unilaterally close a border. This is formal, and it lives mostly in news, politics, law, and business writing — you would not say it to a friend about lunch. It usually carries a small complaint inside it: the person saying it often feels the other side had a right to be asked and was not. So "they changed the terms unilaterally" is not neutral reporting; it is a quiet accusation. Sometimes, though, it is simply a description of a legal fact — a "unilateral" contract or a unilateral withdrawal — with no blame attached, and you tell the difference from who is speaking and why.