An air-fuel ratio is a dimensionless number that expresses the ratio between the actual amount of air in the mixture and the theoretical amount (stoichiometric amount) of air corresponding to the fuel used. The coefficient, named in English as the Air-Fuel Ratio, is denoted by the Greek letter λ (lambda).

The coefficient defined in this way is mainly used in the field of internal combustion engines because it is a necessary parameter for the preparation of the mixture. The air-fuel ratio thus thoroughly expresses the mixture's richness level. Each fuel needs a different amount of air for its perfect combustion.