Reliable messaging is the concept of communicating messages across an unreliable infrastructure whilst being able to make certain guarantees about the successful transmission of the messages; for example, that if the message is delivered, it is delivered at most once, or that all messages successfully delivered arrive in a particular order.
One protocol that implements this concept is WS-ReliableMessaging, which handles reliable delivery of SOAP messages.
Reliable delivery can be contrasted with best-effort delivery, where there is no guarantee that messages will be delivered quickly, in order, or at all. A reliable delivery protocol can be built on an unreliable protocol; an extremely common example is the layering of Transmission Control Protocol on the Internet Protocol, a combination known as TCP/IP.