A "dead bus" is defined to be a bus that has no current/voltage from any source to power the load(s) that is(are) connected to it.
In some plants there is a tie-line breaker downstream of the generator breaker(s) which connects the output of the plant with a larger grid and other generators and their prime movers. If the generator(s) in the plant are not running, or were also tripped at the time the tie-line breaker opened, the plant would be said to be "black". And the bus between the generator breaker(s) and the tie-line breaker is said to be "dead."
If there is an emergency generator which can be started and used to start the other generator prime mover(s), or if one or more of the generator prime movers is capable of what's called "black start" (without AC from any source) the prime mover can be started and the generator accelerated to rated speed. At that time, there is no voltage on the "line/load" side of the generator breaker, a "dead bus"--so there is nothing to synchronize to. If the generator control scheme is capable of what's called 'dead bus breaker closure' then the generator breaker can be closed to energize the bus between the generator breaker and the tie-line breaker, and this will usually supply the plant "house" or "hotel" load until the tie-line breaker can be re-closed.
In some high-voltage distribution switch-yards which feed multiple circuits (buses) it's possible that one or more of the bus breakers tripped for some reason, and the loads supplied by that(those) breaker(s) are now without power. That would also be considered a dead bus--a circuit without power from any source.