An intermediary is someone who acts as a go-between or a mediator between two other people. Be careful when you're the intermediary between two friends who are fighting, because they might both end up mad at you!
The word intermediary comes from the Latin intermedius, which is also the root word for intermediate. Inter- means between, and medius means the middle — intermediary retains that sense of being in the middle. Intermediaries are used to negotiate between two countries who are at odds, between a company and a client over a contract, between two bickering children, or between a boss and an employee in salary negotiations.