Smoke is a suspension in air (aerosol) of small particles resulting from incomplete combustion of a fuel. It is commonly an unwanted by-product of fires (including stoves and lamps) and fireplaces, but may also be used for pest control (cf. fumigation), communication (smoke signals), defence (smoke-screen) or inhalation of tobacco or other drugs. Smoke is also sometimes a component of internal combustion engine exhaust gas, particularly diesel exhaust.

Smoke inhalation is the primary cause of death in victims of indoor fires. The smoke kills by a combination of thermal damage, poisoning and pulmonary irritation caused by carbon monoxide, cyanide and other combustion products.

Smoke particles are actually an aerosol (or mist) of solid particles or liquid droplets that are close to the ideal range of sizes for Mie scattering of visible light. This effect has been likened to three-dimensional textured privacy glass—the smoke cloud does not obstruct an image, but thoroughly scrambles it.