An atmosphere is a layer or layers of gases that surrounds a material body in space [Source]. The body must be of sufficient mass and gravity to keep the gases from escaping into space. An atmosphere can also mean the gases trapped in an artificial pressurized habitat (paraterraforming).
Importance of an Atmosphere
Because no known life form can exist in the void of space for very long all terraformed worlds would need an atmosphere of at least 15-30% Oxygen, 60-75% Nitrogen, and <1% carbon dioxide, to breathe in and support carbon and nitrogen cycles. An atmosphere would also be necessary to heat up the surface of the planet, by means of Greenhouse Gases.