A process in which some exhaust gas is sent to the
intake manifold to be mixed in with incoming air, used in
internal combustion engines.
The idea is that by replacing some of the highly volatile fuel/air mixture with inert exhaust gas, the resulting combustion is slower and cooler, which causes more complete combustion and consequently lower harmful emissions.
The EGR Valve regulates the amount of exhaust gasses that are mixed back in.