Can someone with english as his first language please give me the definition of HOT BUS?
by definition, a hot bus is always powered. Any energized bus is hot, but the common use term of a "hot bus" doesn't need a switch to be powered, and comes directly from the power source.
A hot bus is a powered bus, but the term is generally used to describe specifically a bus that can't be powered down, or shut down as other normal busses (but still employs circuit breaker protection). If the aircraft battery master switch is shut off while sitting on the ground on battery power, for example, causing the entire aircraft's electrical system to be shut down, the only bus remaining powered is generally the hot battery bus.
Some airplanes put certain critical items on the hot battery bus, or items which are needed when the rest of the system isn't powered. Door latch motors, for example, may be one such item. In some aircraft, pitot heat comes off the hot battery bus. What's specifically on the hot battery bus depends on the airplane under discussion.