Pressure required to overcome resistance to passage of exhaust gases through the manifold, muffler and exhaust outlet pipes; increased back pressure can reduce engine power output, cause burned exhaust valves.
In an air flow, the resistance at any point of the remainder of the downstream circuit or of any nominated sections of the circuit. In liquid filtration the back pressure can be due to a hydrostatic head, e.g. pumping from a filter to a tank at a higher level. Back pressure is also applied to keep CO2 in solution in the filtration and handling of carbonated beverages.
This is the refrigerant pressure in the low side of the system also called low side pressure or suction pressure.