The Free On-line Dictionary of Computing (30 December 2018):
A part of an Ethernet or other network, on
which all message traffic is common to all nodes, i.e. it is
broadcast from one node on the segment and received by all
others. This is normally because the segment is a single
continuous conductor, though it may include repeaters(?).
Since all nodes share the physical medium, collision
detection or some other protocol is required to determine
whether a message was transmitted without interference from
other nodes. The receiving node inspects the destination
address of a packet to tell if it was (one of) the intended
Communication between nodes on different segments is via one
or more routers.