Definition - What does Atmosphere mean?
Atmosphere describe the mass of gases which surround a planet or star. They act as an envelope around our planet and others. The earth's atmosphere is the air we breathe in our day to day lives and is regulated by living organisms on our planet.
Safeopedia explains Atmosphere
Gaseous atmospheres surround planets because of their weight. Due to the laws of gravity, all matter is drawn toward other matter and forms into large masses that are planets and stars. The denser and heavier solid materials end up in the centre of the mass and the lighter materials like water and gases rise to the top much like oil floating on water.