1) In general, an intermediary is a person or service that is involved as a third party between two or more end points in a communication or transaction. For example, someone may attempt to get two sides together in a labor dispute (this task is sometimes known as mediation) or a real estate broker arranges for buyer and seller to complete a real estate transaction.
On the World Wide Web, the role of an intermediary is sometimes said to be no longer necessary as, for example, when you look up legal or medical information directly yourself or when you share files with other Internet users directly (see peer-to-peer). This idea is sometimes known as disintermediation. Any attempt by the third party to discourage this or to encourage use of the third party's services is known as antidisintermediation.
2) An intermediary is a program or set of programs that in some way evaluates, filters, modifies, or otherwise interjects some function between two end users or end-use programs (such as a client/server). An example is the proxy server that most companies place between their internal Web users and the public Internet. At IBM Research, a formal structure for developing, distributing, and managing this kind of intermediary has been created and is known as Web Intermediaries.