A term used in automotive circles to describe the "ambient" operating pressure of the exhaust system (i.e. the amount of pressure the engine has to fight against to expel the exhaust). Back pressure is increased by restrictive exhaust pipes and turbochargers. One would think that lowering the pressure the engine has to fight against would increase the mechanical efficency and thus the power output. This is usually true, and is the reason why a performance enthusiast will readily spend hundreds, even thousands of dollars on performance headers and free-flowing exhaust systems.
However, engines need some back pressure to work properly. A small amount of back pressure helps the engine expel the exhaust in a uniform fashion. A good analogy someone gave me uses a drinking straw as an example. Drinking through a coffee stirrer represents a restrictive exhaust with a lot of back pressure. It is difficult to drink through a coffee stirrer, as it is difficult for an engine to expel exhaust through a restrictive system. A normal drinking straw represents a good free-flowing exhaust. It is easy to drink through a regular sized straw, as it is easy for an engine to expel exhaust through a good exhaust system. However, if the straw is made too big, it becomes more difficult to drink through it. This represents a lack of back pressure.