define carbon dioxide
carbon dioxide - a heavy odorless colorless gas formed during respiration and by the decomposition of organic substances; absorbed from the air by plants in photosynthesis.
uk â us â also carbon, abbreviation CO2. âº ENVIRONMENT the gas formed when carbon is burned, or when people or animals breathe out. Carbon dioxide is a greenhouse gas: The UK plans to reduce carbon dioxide emissions by 60% by 2050. Hotter days could be ahead as the carbon dioxide levels in Earth's atmosphere continue to rise.