An atmosphere was a gaseous envelope surrounding planets, such as Earth.
Space had no atmosphere, so people needed to wear spacesuits when in outer space. (TV: Enlightenment)
Earth
Earth, and later on, the terraformed Mars' atmosphere, was 80% nitrogen and 20% oxygen. (AUDIO: The Resurrection of Mars) New Earth had the same atmosphere as Earth, which was why humans made it their new home after Earth was destroyed. (TV: New Earth)
Due to human activity, Earth had a lot of toxins and dioxins in its atmosphere, which would have provided decent nutrition for the Nestene Consciousness. (TV: Rose)
Earth's atmosphere had been contaminated after the fall of Satellite Five. (TV: Bad Wolf)
During the Year That Never Was, Earth's atmosphere was temporarily opened to unleash an army of Toclafane. (TV: The Sound of Drums)
In 2009, the Tenth Sontaran Battle Fleet made Luke Rattigan develop the ATMOS in cars across the entire planet Earth. This was part of their plan to poison Earth's atmosphere with clone feed, so Earth could become the new clone world for the Sontaran species. The Tenth Doctor later cleared the atmosphere, saving the planet. (TV: The Poison Sky)
In a parallel world, Earth's atmosphere was poisoned by the Sontarans and then cleared by Torchwood Three. (TV: Turn Left)
Atmospheres of other worlds
The Tenth Doctor needed a spacesuit on Mars as it had a poor atmosphere. (TV: The Waters of Mars)
Alfava Metraxis had a breathable atmosphere. (TV: The Time of Angels)
Elliot Sardick was able to control the atmosphere of his home planet, allowing him to release the sky fish that resided there into the air beneath at his discretion. (TV: A Christmas Carol)
The planet Malcassairo had an atmospheric shell. (TV: Utopia)