a particular environment or surrounding influence
"there was an atmosphere of excitement"
a unit of pressure: the pressure that will support a column of mercury 760 mm high at sea level and 0 degrees centigrade
"The engineer adjusted the gauge to ensure the atmospheric pressure remained exactly one atmosphere during the calibration test."
the mass of air surrounding the Earth
"there was great heat as the comet entered the atmosphere"
"it was exposed to the air"
the weather or climate at some place
"the atmosphere was thick with fog"
the envelope of gases surrounding any celestial body
"The spacecraft launched successfully, escaping Earth's atmosphere to begin its journey toward Mars."
a distinctive but intangible quality surrounding a person or thing
"an air of mystery"
"the house had a neglected air"
"an atmosphere of defeat pervaded the candidate's headquarters"
"the place had an aura of romance"
The gases surrounding the Earth or any astronomical body.
"Scientists are working hard to determine if Mars ever had an atmosphere thick enough to support liquid water on its surface."
In plain English: Atmosphere is the layer of gases that surrounds a planet, like Earth.
"The warm atmosphere in the room made everyone feel welcome."
Usage: Use "atmosphere" to describe the layer of gases surrounding a planet or celestial body. In everyday conversation, it often refers to the mood or feeling of a specific place rather than its scientific composition.
The word "atmosphere" comes from the French atmosphère and ultimately from Ancient Greek, where it combined atmós, meaning "steam," with sphaîra, meaning "sphere." It entered English to describe the layer of gases surrounding the Earth.