A cosignatory is one of the people who put their name on the same official paper — a contract, a treaty, or another legal document. They are usually signing for a group, not only for themselves: a company, a government department, a country, and so on. If three countries all sign a deal, each country (or the person signing for it) is a cosignatory. The word is stiff and official. You will meet it in news about countries and in legal writing, not at the dinner table. Though actually, people sometimes mix it up with cosigner. A cosigner on a loan is the friend or parent who promises to pay if you cannot. A cosignatory is one of the named parties who sign the same paper, usually as equals. The word can also sit in front of a noun: the cosignatory states means the states that signed together.

共同簽署人（跟貸款的連帶保證人 cosigner 不是同一個字。）