A chimney is an architectural ventilation structure made of masonry, clay or metal that isolates hot toxic exhaust gases or smoke produced by a boiler, stove, furnace, incinerator or fireplace from human living areas. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the stack, or chimney effect. The space inside a chimney is called the flue. Chimneys are adjacent to large industrial refineries, fossil fuel combustion facilities or part of buildings, steam locomotives and ships.

A vertical chimney erected on the mechanical penthouse of a residential high rise in Ontario, Canada for ejecting combustion products from the building's water boiler.
Smokestacks in Manchester, England c. 1858 watercolor by William Wyld
A chimney by the old fire station near Culture Center Vernissa in Tikkurila, Vantaa, Finland
The world's tallest chimney, of the GRES-2 power plant in Ekibastuz, Kazakhstan, stands 419.7 m (1,377 ft) tall.
A chimney remaining after the destruction of a 19th-century two-story house (Mount Solon, Virginia).

In the United States, the term 'Smokestack industry' refers to the environmental impacts of burning fossil fuels by industrial society including the electric industry during its earliest history. The term smokestack (colloquially, stack) is also used when referring to locomotive chimneys or ship chimneys, and the term funnel can also be used.[1][2]

The height of a chimney influences its ability to transfer flue gases to the external environment via stack effect. Additionally, the dispersion of pollutants at higher altitudes can reduce their impact on the immediate surroundings. The dispersion of pollutants over a greater area can reduce their concentrations and facilitate compliance with regulatory limits.