The Free On-line Dictionary of Computing (30 December 2018):

protocol layer
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)