evaporation

The term evaporation refers to a liquid turning to a gas but also is used in an analogous sense in astrophysics: evaporation from an atmosphere or cloud refers to a constituent particle escaping gravity and flying free, either for being in the far end of the particles' velocity distribution (for which a timescale may be devised, e.g., the effectively-short timescale for hydrogen to escape Earth's atmosphere) or through addition of energy, e.g., photoevaporation. For a planet's atmosphere, such evaporation would constitute atmospheric escape.

The term evaporation is also used analogously in the study of gravitationally-bound groups of stars (galaxies, globular clusters, open clusters) to indicate the escape of stars that have by some means acquired sufficient velocity.