Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material.
[1] Outgassing can include sublimation and evaporation (which are phase transitions of a substance into a gas), as well as desorption, seepage from cracks or internal volumes, and gaseous products of slow chemical reactions.
NASA and ESA maintain lists of materials with low-outgassing properties suitable for use in spacecraft, as outgassing products can condense onto optical elements, thermal radiators, or solar cells and obscure them.
NASA's Stardust space probe suffered reduced image quality due to an unknown contaminant that had condensed on the CCD sensor of the navigation camera.
[2] A similar problem affected the Cassini space probe's Narrow Angle Camera, but was corrected by repeatedly heating the system to 4 °C.
At the Earth's tectonic divergent boundaries where new crust is being created, helium and carbon dioxide are some of the volatiles being outgassed from mantle magma.