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

root bridge

    A bridge which
   continuously transmits network topology information to
   other bridges, using the spanning tree protocol, in order to
   notify all other bridges on the network when topology changes
   are required.

   This means that a network is able to reconfigure itself whenever a
   network link (e.g. another bridge) fails, so an alternative path
   can be found.  The presence of a root bridge also prevents loops
   (network loop) from forming in the network.

   The root bridge is where the paths that frames take through
   the network they are assigned.  It should be located centrally
   on the network to provide the shortest path to other links on
   the network.  Unlike other bridges, the root bridge always
   forwards frames out over all of its ports.

   Every network should only have one root bridge.  It should
   have the lowest bridge ID number.

   (2000-11-26)