Definition
The primary electrical distribution bars in a turboprop airplane that receive power from the generators and feed it onward to the airplane's electrical systems and subsidiary buses. In most turboprop installations there are two main buses, one associated with each generator, and they can typically be tied together so either generator can power both sides if the other fails.
Plain English
The two big electrical highways inside the airplane. Power comes in from the generators, lands on these highways, and from there it flows out to everything else that needs electricity.
Context Anchor
Seen in turboprop electrical system descriptions, especially when explaining how generator or battery power is distributed through the airplane.
Derivation
In electrical engineering, 'bus' is short for 'busbar' -- a metal bar that distributes current to multiple circuits. 'Main' simply marks these as the primary distribution points, as opposed to smaller secondary buses that branch off them.
Why Pilots Care
Knowing which systems are on which main bus helps a pilot isolate failures and decide what can be shed to keep essential power available.
Intuition Check
Do not think of buses as vehicles here. In an electrical system, a bus is a power distribution point; main buses are the primary ones.
Example Sentence 1
After the left generator failed, the crew tied the buses so the right generator could power both main buses.
Example Sentence 2
The pilot isolated the faulty circuit by pulling breakers on the right main bus while keeping the left main bus online.