In network topology, a bus is a medium that is physically shared, or uses interconnection that makes the logical equivalent of a shared medium. All connected stations have equal access to the medium. Modern busses tend to be full-duplex, although they achieve this with a physical path in each direction.
The medium access control for a bus can be contention-based, as for IEEE 802.3, or token-based/master-slave as for MIL-STD-1553.
While wireless media are indeed shared access, the term "bus" implies a wired medium.