According to the definitions, is there any difference between Jettison and Dump fuel? Does one of them use a different procedure, or they are simply synonyms?
In the context of fuel, jettisoning fuel would imply that the fuel is in a container that can be detached while in flight. In the linked image, the large tank below the main body is an external fuel pod that can be jettisoned. Jet with external fuel tank I believe in WWII that P51s would have drop tanks for escorts that would be jettisoned once an enemy encounter was expected.
Dumping fuel would simply mean that there is a type of valve located on the fuel tanks that can be actuated in some way such that the fuel can freely exit. Jetliner dumping fuel
To say differently, if one was holding water in a bucket, pouring the water on the ground would be 'dumping fuel' and simply dropping the bucket would be 'jettisoning fuel'
Reviewing the wikipedia article on Fuel Dumping, the author(s) make no such distinction between
jettisoning and seem to use
In the context of ATP, after reading the wikipedia article on fuel dumping, I believe that they are considered synonyms. I believe a non-pilot would go more along with my definition however.
I answered in this manner as I believe that the OP is not a native English speaker and that clarification was needed as to distinguishing the two as they seem very similar. I am a native English speaker, but lay no claim to being a master of it.
Jettison means to throw or drop (something) from an aircraft. Dumping fuel means the fuel (alone) is jettisoned. One can jettison any item (removable in flight) from the aircraft.
For example, a combat aircraft jettisons its fuel tank, while a civil airliner dumps the fuel, not the tank.
Jettison means to 'throw something overboard', it isn't limited to fuel. To take a practical example, if I experienced engine failure in a light aircraft and had to make an emergency landing (especially a ditching), I might jettison any large or heavy items in the cabin to avoid them causing injuries during the landing. This appears in the ditching checklist for the C172, for example (my emphasis):
Radio -- TRANSMIT MAYDAY on 121.5 MHz, giving location and intentions and SQUAWK 7700.
Heavy Objects (in baggage area) -- SECURE OR JETTISON (if possible).
Therefore, dumping fuel is one specific type of jettisoning where only the fuel is jettisoned.