A state of a solution or compound in which it is a gaseous suspension of very fine solid or liquid particles. Aerosol comes from the longer term 'aero-solution' meaning, quite obviously, a solution mixed with air.
Most aerosol prodcts come in the now familiar 'can' form, the liquid contents inside being under pressure from compressed gasses held in the container with them. When a valve on the container is opened, the compressed gasses inside the can push the liquid out at a high rate of speed such as they turn into a mist, and thus are aerosol, or 'mixed with air'.