According to w3c.org, it says "WSDL is XML format describing network services as a set of endpoints operating on messages either document-oriented or procedure-oriented information."
But according to http://en.wikipedia.org/wiki/List_of_web_service_protocols, it says "The following is a list web service protocols: SOAP, REST... WSDL"
I wouldn't call it a protocol on its own, but it has some elements which you could classify under message exchange (or communication). In WSDL 2.0 it is known under the Message Exchange Pattern (MEP): "A message exchange pattern is a template for the exchange of one or more messages, and their associated faults, between the service and one or more other nodes as described by an Interface Operation component.". In WSDL 1.0 you can compare it with the four fixed "transmission primitives": one-way, request-response, solicit-response and notification.
WSDL is part of the "Web Services Protocol Stack" which tries to make a layered presentation of the elements playing part in the communication between applications. When you think of a network protocol stack you will think of the ISO/OSI stack. A nice and theoretical comparison is described by one of the members of W3C: Identical Principles, Higher Layers: Modeling Web Services as Protocol Stack.