気体中に浮遊する微小な液体または固体の粒子と周囲の気体の混合体をエアロゾル(aerosol)と言います。エアロゾル粒子は，その生成過程の違いから粉じん(dust)とかフューム(fume)，ミスト(mist)，ばいじん (dust) などと呼ばれ，また気象学的には，視程や色の違いなどから，霧(fog)，もや(mist)，煙霧 (haze)，スモッグ(smog)などと呼ばれることもあります。エアロゾル粒子の性状は，粒径や化学組成，形状，光学的・電気的特性など多くの因子によって表され，きわめて複雑です。
その上，例えば粒径についていえば，分子やイオンとほぼ等しい0.001μm＝1nm程度から花粉のような100μm程度まで約5桁にわたる広い範囲が対象となり，また個数濃度についても，清浄空気の10個/cm3程度から発生源近傍の10の6乗～10の10乗個/cm3程度まで 7～8 桁にもわたり，さらに最近の超クリーンルームにおいては10の-5乗個/cm^3程度まで要求されるようになっています
Aerosol refers to a mixture of tiny liquid or solid particles suspended in a gas. Aerosol particles are referred to as dust, fume, mist, or particulates, depending on their generation process. In meteorology, they may also be called fog, mist, haze, or smog based on visibility and color differences. The properties of aerosol particles are represented by many factors, including particle size, chemical composition, shape, and optical and electrical characteristics, making them extremely complex.
For example, regarding particle size, the range can vary widely from approximately 0.001 μm (1 nm), which is nearly equal to that of molecules or ions, to about 100 μm, similar to pollen, covering about five orders of magnitude. Additionally, the number concentration can range from about 10 particles/cm³ in clean air to around 106 to 1010 particles/cm³ near sources of emission, spanning 7 to 8 orders of magnitude. Furthermore, in recent ultra-clean rooms, requirements have risen to about 10-5 particles/cm³.