A grouping of services that have power available to their protective devices regardless of cockpit switch positions.
The only way to remove power from a hot bus is to remove all power sources and physically disconnect the battery (or cause it to become flat
).
It can include services that are critical, services that are a final back-up and/or services that are usefull for ground servicing eg the refueling pannel, some lights etc.
In the simplest terms;
A Bus is simply a grouping of services.
A hot bus is a grouping of services that constantly has power.
So another definition could be- a grouping of services that can not be load-shed.