A specific group of electrical wires that carries a particular group of voltages or signals between devices. The term is borrowed from computer engineering. The power bus could also be compared to the busbars, heavy copper bars, used to conduct electricity at high currents in the electrical industry.
An example would be the power bus that supplies voltage and current to the track, running in parallel with the track and connected at multiple points using feeders.
Another example would be something like LocoNet, which connects various devices together into a network, allowing communication between devices.
A common misspelling is buss. The correct spelling is bus.