The gaseous envelope surrounding the *Earth*, and retained by the Earth's gravitational field. If your question is about the atmosphere on another celestial body or is more astronomy related, please ask on Astronomy.SE.
The atmosphere is the gaseous outer layer of a planet. For the Earth (and other inner planets), it is characterised by a sharp lower boundary, separating it from the ocean, the cryosphere, or land surface. The atmosphere is a highly dynamical system due to differential heating, effects of the rotating Earth, transport of latent and sensible heat, and other effects. For more information: