42 U.S.C. § 7501

For the purpose of this part— (1) Reasonable further progress .— The term “reasonable further progress” means such annual incremental reductions in emissions of the relevant air pollutant as are required by this part or may reasonably be required by the Administrator for the purpose of ensuring attainment of the applicable national ambient air quality standard by the applicable date. (2) Nonattainment area .— The term “nonattainment area” means, for any air pollutant, an area which is designated “nonattainment” with respect to that pollutant within the meaning of section 7407(d) of this title . (3) The term “lowest achievable emission rate” means for any source, that rate of emissions which reflects— (A) the most stringent emission limitation which is contained in the implementation plan of any State for such class or category of source, unless the owner or operator of the proposed source demonstrates that such limitations are not achievable, or (B) the most stringent emission limitation which is achieved in practice by such class or category of source, whichever is more stringent. In no event shall the application of this term permit a proposed new or modified source to emit any pollutant in excess of the amount allowable under applicable new source standards of performance. (4) The terms “modifications” and “modified” mean the same as the term “modification” as used in section 7411(a)(4) of this title .