Offset

A method used in the 1990 Clean Air Act to give companies which own or
operate large (major) sources in nonattainment areas flexibility in meeting
overall pollution reduction requirements when changing production
processes.  If the owner or operator of the source wishes to increase
release of a criteria air pollutant, an offset (reduction of a somewhat greater
amount of the same pollutant) must be obtained either at the same plant or by
purchasing offsets from another company.