1. Extremely-fine liquid droplets or solid particles that remain suspended in air as fog or smoke. These non-gaseous microscopic bits (with radius from one-hundredth to one ten-millionth of a centimeter, or 100 to 0.001 micrometer) are produced from natural and artificial sources, and usually includes mineral dust, pollen, and sulfuric acid. While aerosols play an important roles in the atmosphere (such as in formation of rain drops, snow flakes, and ice crystals), they also cause climatic effects similar to those produced by greenhouse gases.
2. Non-refillable packaging used in dispensing certain consumer, industrial, and pharmaceutical products. It consists usually of a metal container which holds a solution (or colloidal suspension, such as paint) and a compressed propellant (such as air or natural gas), and is topped by a mechanical valve. When the valve is opened, the solution (depending on its components and type of valve) is forced out as a fine or coarse spray, foam, or gel.