Source : Free On-Line Dictionary of Computing

protocol layer
     
         The {software} and/or {hardware} environment of
        two or more communications devices or computers in which a
        particular {network} {protocol} operates.  A network
        connection may be thought of as a set of more or less
        independent protocols, each in a different layer or level.
        The lowest layer governs direct host-to-host communication
        between the hardware at different hosts; the highest consists
        of user {application programs}.  Each layer uses the layer
        beneath it and provides a service for the layer above.  Each
        networking component {hardware or software} on one host uses
        {protocols} appropriate to its layer to communicate with the
        corresponding component (its "peer") on another host.  Such
        layered protocols are sometimes known as peer-to-peer
        protocols.
     
        The advantages of layered {protocols} is that the methods of
        passing information from one layer to another are specified
        clearly as part of the {protocol} suite, and changes within a
        protocol layer are prevented from affecting the other layers.
        This greatly simplifies the task of designing and maintaining
        communication systems.
     
        Examples of layered protocols are {TCP/IP}'s five layer
        {protocol stack} and the {OSI} seven layer model.
     
        (1997-05-05)