A common _Messaging Infrastructure_ or _Message Bus_ enables secure and reliable communication across enterprise networks. The message bus has the ability to _route_ messages between message publishers and message subscribers. It queues messages if a subscriber is not available, relieving the publisher from the need to store and resend messages.
Applications publish data to or receive data from the message bus via _Adaptors_ or _Connectors_. These connectors make an application's proprietary inteface available to other applications via the message bus.
Since EAI solutions act as the glue that holds together many disparate systems, they must provide interfaces or adapters through which each of these systems can be invoked and accessed. The Adapters translate between bus messages and the application's API and back again.
See also: http://www.eaipatterns.com/