Carbon dioxide (chemical formula: ) is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state. It is currently at a globally averaged concentration of approximately 383 ppm by volume in the Earth's atmosphere, although this varies both by location and time. Carbon dioxide is an important greenhouse gas because it transmits visible light but absorbs strongly in the infrared.

Carbon dioxide is produced by all animals, plants, fungi and microorganisms during respiration and is used by plants during photosynthesis. This is to make sugars which may either be consumed again in respiration or used as the raw material for plant growth. It is, therefore, a major component of the carbon cycle. Carbon dioxide is generated as a byproduct of the combustion of fossil fuels or vegetable matter, among other chemical processes. Inorganic carbon dioxide is output by volcanoes and other geothermal processes such as hot springs.

Carbon dioxide has no liquid state at pressures below 5.1 atm, but is a solid at temperatures below -78 °C. In its solid state, carbon dioxide is commonly called dry ice.

CO 2 is an acidic oxide: an aqueous solution turns litmus from blue to pink.