The atmosphere refers to the layer of gases that surrounds the Earth (or any other celestial body). The Earth's atmosphere is made up of 78% nitrogen, 21% oxygen and trace amounts of other gases, such as carbon dioxide, water vapor, and argon. Theses gases held by gravity around Earth and around other planets. So "Atmosphere" can also be used to talk about gases around stars.
The atmosphere is important for a number of reasons. It helps to regulate the Earth's temperature by trapping heat from the sun, and it also protects the planet from harmful solar radiation and meteorites. In addition, the atmosphere plays a vital role in supporting life on Earth, as it provides the oxygen that plants and animals need to survive.
In the aerospace industry, the atmosphere is an important factor to consider when designing and operating aircraft and spacecraft. For example, aircraft must be designed to be able to withstand the stresses of flying through the atmosphere at high speeds, and spacecraft must be able to navigate through the thin upper atmosphere when entering or leaving the Earth's orbit. Engineers also study the Earth's atmosphere in order to better understand weather patterns and climate, and to develop ways to monitor and protect the planet's environment.