Back pressure is the small amount of vacuum that is created when the used steam condenses from the turbine back into re-usable water.

Suction pressure

Pressure going in the opposite direction in which it should be going.

depending on its design, a resonator will produce anywhere from almost no back pressure, to a mild amount.

Engine back pressure is a common ailment in faulty engines. It is when the exhaust sends the power to the very back of the vehicle, instead of through its engine.