40 CFR § 51.1400 - Definitions.
---
identifier: "/us/cfr/t40/s51.1400"
source: "ecfr"
legal_status: "authoritative_unofficial"
title: "40 CFR § 51.1400 - Definitions."
title_number: 40
title_name: "Protection of Environment"
section_number: "51.1400"
section_name: "Definitions."
chapter_name: "ENVIRONMENTAL PROTECTION AGENCY"
subchapter_number: "C"
subchapter_name: "AIR PROGRAMS"
part_number: "51"
part_name: "REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL OF IMPLEMENTATION PLANS"
positive_law: false
currency: "2026-04-05"
last_updated: "2026-04-05"
format_version: "1.1.0"
generator: "[email protected]"
authority: "23 U.S.C. 101; 42 U.S.C. 7401-7671q."
regulatory_source: "36 FR 22398, Nov. 25, 1971, unless otherwise noted."
cfr_part: "51"
---
# 51.1400 Definitions.
The following definitions apply for purposes of this subpart. Any term not defined herein shall have the meaning as defined in § 51.100.
*Attainment year* means the calendar year in which the attainment year ozone season occurs.
*Attainment year ozone season* means the full ozone season immediately preceding a nonattainment area's maximum attainment date.
*CAA* means the Clean Air Act as codified at 42 U.S.C. 7401-7671q (2010).
*Former attainment date* means any attainment date associated with the classification under subpart 2 of part D of title I of the CAA preceding reclassification from a lower classification to a higher classification.
*Former classification* means any classification under subpart 2 of part D of title I of the CAA preceding reclassification from a lower classification to a higher classification.
*Higher classification/lower classification* means for purposes of determining which classifications are higher or lower, the classifications are ranked from lowest to highest as follows: Marginal; Moderate; Serious; Severe-15; Severe-17; and Extreme.
*I/M* refers to the inspection and maintenance programs for in-use vehicles required under the 1990 CAA Amendments and defined by subpart S of 40 CFR part 51.
*Initially classified* means the first nonattainment classification that becomes effective for an area for a specific ozone NAAQS and does not include reclassification to another classification for that specific NAAQS.
*Initially designated* means the first designation to nonattainment that becomes effective for an area for a specific ozone NAAQS.
*Ozone season* means for each state (or portion of a state), the ozone monitoring season as defined in 40 CFR part 58, appendix D, section 4.1(i) for that state (or portion of a state).