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a resistant pressure exerted by liquid or gas against the forward motion or flow of an exhaust or pipe system:
Careful control of backpressure ensures an even supply of oil from the well.
Historical Examples

Its use is to show the amount of back pressure, and in a condensing engine to show the degree of vacuum obtained.
Modern Machine-Shop Practice, Volumes I and II Joshua Rose

But the back pressure of the cat’s body overloaded the discharge circuits.
Survival Tactics Al Sevcik

The back pressure saved will at once show in the pulling power of the engine, and every one will be surprised.
Farm Engines and How to Run Them James H. Stephenson

There is then no back pressure on the exhaust steam, theoretically.
Farm Engines and How to Run Them James H. Stephenson

So in using steam expansively the back pressure must be at a minimum, and this is especially true in the compound engine.
Farm Engines and How to Run Them James H. Stephenson

He opened the muffler so as to get rid of all back pressure, and opened the throttle to its widest extent.
Bert Wilson at the Wheel J. W. Duffield

The problem now is to get rid of that back pressure, and the old type engines give a hint how it may be done.
Motors James Slough Zerbe

As Bert had already explained to Dick, this was done to eliminate any back pressure from the exhaust gases.
Bert Wilson at the Wheel J. W. Duffield

There is no back pressure of blood here, so that the movement of fluid through the lymphatics is not hindered.
Physiology Ernest G. Martin

The line from d to e is therefore the exhaust line, and that from e to a is the back pressure line or counter pressure line.
Modern Machine-Shop Practice, Volumes I and II Joshua Rose

noun
(engineering)

the pressure that opposes the motion of a piston on its exhaust stroke in an internal-combustion engine
the exhaust pressure in external combustion engines

(med) the local pressure that builds up when fluid flow is obstructed in the cardiovascular or urinary systems

back pressure n.
Pressure exerted upstream in the circulation as a result of obstruction to forward flow, as when congestion in the pulmonary circulation results from failure of the left ventricle.