An atmosphere is a layer of gases surrounding a celestial body, like Earth. It is essential for sustaining life, providing necessary elements such as oxygen for respiration and carbon dioxide for photosynthesis. The Earth's atmosphere is composed mainly of nitrogen (78%) and oxygen (21%), with trace gases like argon and carbon dioxide contributing to various meteorological and climatic processes. It also plays a critical role in protecting life from harmful solar radiation and maintains the planet's temperature through the greenhouse effect.