1. The loss of gas from within a planet through processes such as volcanism. Outgassing is thought to have been responsible for much of the atmospheres of Earth and the other terrestrial planets.

2. In a more general sense, the loss of gas from any solid, such as the outgassing and recondensation of water vapour and other light molecules from spacecraft structures. This form of outgassing can have serious effects on spacecraft mechanisms and telescope optics.