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

multicast backbone

   (MBONE) A virtual network on top of the Internet which
   supports routing of IP multicast packets, intended for
   multimedia transmission.  MBONE gives public access desktop
   video communications.  The quality is poor with only 3-5
   frames per second instead of the 30 frames per second of
   commercial television.  Its advantage is that it avoids all
   telecommunications costs normally associated with
   teleconferencing.  An interesting innovation is the use of
   MBONE for audio communications and an electronic "whiteboard"
   where the computer screen becomes a shared workspace where two
   physically remote parties can draw on and edit shared
   documents in real-time.

   (1994-10-27)