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 recipient(s).
Communication between nodes on different segments is via one or more routers.