When measuring gases like carbon monoxide, carbon dioxide, oxygen, or methane, the term concentration is used to describe the amount of gas by volume in the air. A common unit of measurement is parts-per-million (abbreviated PPM).
Parts-per-million is the ratio of one gas to another. For example, 1,000 PPM of CO2 means that if you could count a million gas molecules, 1,000 of them would be of carbon dioxide and 990,000 molecules would be some other gases.