Outgassing is the process by which molecules of gas trapped within a solid work their way out of the solid and into the surrounding environment.
This can happen slowly, as in building materials, rocks, and most plastics. Some materials can outgas very lethal compounds; for example, plywood has been known to leak formaldehyde. This is not good, especially in areas with insufficient ventilation.
It can also happen very quickly, when the pressure difference between the inside of the solid and the environment is very great - for example, in outer space. Molecules blasting out of parts at very high speeds is not a happy thing for the space industry.
Various coatings have been developed to counter the effects of outgassing. Special non-outgassing foams and plastics, such as Delrin®, have also been developed for use in space and other situations requiring minimal outgassing.