In case you didn't mean the term to apply to air-conditioners.
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EGR (Exhaust Gas Recirculation)
The purpose of the EGR port is to feed exhaust gas from the exhaust system back into the intake air flow whenever the coolant temperature is high enough and the ported vacuum is also high (ie, not during startup and only during heavy loads or hard acceleration). This decreases the engine temperature, which in turn decreases the emission of harmful nitrous oxide (NOx) gases. Reduced temps will reduce pinging, which will in turn allow you to advance your timing farther for better power throughout most of the RPM range. By diluting your intake charge with exhaust gas, the EGR will reduce your peak horsepower at higher RPM's, but it will not adversely affect power under normal driving conditions. In fact, with the base ignition timing properly set, the EGR will allow for increased power at low- and mid-range RPM's.

http://www.jedi.com/obiwan/jeep/misc/vacuumhoses.html