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

physical layer
layer 1

    Layer one, the lowest layer in the OSI seven
   layer model.  The physical layer encompasses details such as
   electrical and mechanical connections to the network,
   transmission of binary data as changing voltage levels on
   wires or similar concepts on other connectors, and data
   rates.

   The physical layer is used by the data link layer.

   Example physical layer protocols are CSMA/CD, token ring
   and bus.

   (2004-06-29)