Air pollutants in sufficient amount to be observable to the human eye or the observation of an emission of opacity or optical density above the threshold of vision.
ref:
https://www.ecfr.gov/current/title-40/chapter-I/subchapter-B/part-49/subpart-C
https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-63/subpart-A/section-63.2
