According to your favorite dictionary (mine is http://www.dictionary.com), "bind" means to tie, fasten or secure. In RPC-style distributed computing, you need to bind a client to a server, which means that you must select the specific server you want to connect to, and then you must determine the specific details required to make the connection. Those specifc details constitute a "binding". It includes the endpoints of the communication, as well as the concrete protocols used to represent the message on the wire and the network protocols used to transfer the message.
In Web services you decribe a binding using the WSDL <binding> element.
This was first published in March 2003