It is a collective name for all greenhouse gases.There are several different types of atmospheric gases that contribute to the greenhouse effect. Fossil combustion primarily gives rise to carbon dioxide, but e.g. cows produce a lot of methane which is also a greenhouse gas. Methane is more short-lived than carbon dioxide but gives almost 30 times higher greenhouse effect. To be able to talk about the effect of all greenhouse gases, they are converted to carbon dioxide equivalents (CO₂e).