1. Fog that has become mixed and polluted with smoke.
2. A form of air pollution produced by the photochemical reaction of sunlight with hydrocarbons and nitrogen oxides that have been released into the atmosphere, especially by automotive emissions.

Word History: New phenomena require new words, so it is not surprising that smog is a relatively recent coinage. The word followed the phenomenon by perhaps half a century, for air pollution was first noticed during the Industrial Revolution. The word smog is first recorded in 1905 in a newspaper report of a meeting of the Public Health Congress. Dr. H.A. des Vɶux gave a paper entitled "Fog and Smoke," in which, in the words of the Daily Graphic of July 26, "he said it required no science to see that there was something produced in great cities which was not found in the country, and that was smoky fog, or what was known as 'smog.'" The next day the Globe remarked that "Dr. des Vɶux did a public service in coining a new word for the London fog."

1. A form of air pollution produced by the reaction of sunlight with hydrocarbons, nitrogen compounds, and other gases primarily released in automobile exhaust. Smog is common in large urban areas, especially during hot, sunny weather, where it appears as a brownish haze that can irritate the eyes and lungs. Ozone, a toxic gas that is not normally produced at lower atmospheric levels, is one of the primary pollutants created in this kind of smog. Also called photochemical smog.
2. Fog that has become polluted with smoke and particulates, especially from burning coal.