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

RG8
thick Ethernet cable

    The original "full spec" cable used for
   10base5 Ethernet networks.  RG8 is stiff, large diameter
   coaxial cable with an impedance of 50 ohms, a member of the
   "Radio Guide" series.  The outer sheath is usually yellow, to
   indicate double shielding, so it is often just called "yellow
   cable".

   10base5 cable is designed to allow transceivers to be added
   while existing connections are live.  This is achieved using a
   "vampire tap".

   RG8 is sometimes called "thicknet" or "thick Ethernet" in contrast
   to RG58, a cheaper, thinner, more flexible alternative.

   (2014-09-06)