Aerosol: 1. In general, a fine mist or spray which contains minute particles. 2. In medicine, a spray administered by a nebulizer and inhaled for treatment. 3. In medicine, a mist that causes disease as, for example, the hantavirus pulmonary syndrome. 4. In the environment, particles emitted into the air naturally as in volcanic eruptions and through human action such as burning fossil fuel. 5. In the environment, the pressurized gas used to propel substances out of a container.