carbon dioxide

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A colorless, odorless, incombustible gas, CO₂, that is formed during respiration, combustion, and organic decomposition, is an essential component in photosynthesis, and is used in food refrigeration, carbonated beverages, inert atmospheres, fire extinguishers, and aerosols.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun inorganic chemistry The normal oxide of carbon, CO2; a colorless, odorless gas formed during respiration and combustion and consumed by plants during photosynthesis.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a heavy odorless colorless gas formed during respiration and by the decomposition of organic substances; absorbed from the air by plants in photosynthesis