The air ratio
Of course there are also operating states beyond lambda = 1. The so-called air ratio provides information about the respective mixture ratio of air and fuel.
If, for example, more fuel is contained in the mixture, it is called a rich mixture - the engine runs with a deficiency of air. The air ratio is lambda < 1.
On the other hand, if there is comparatively too little fuel with a relatively large amount of air, it is referred to as a lean mixture. There is excess air in the combustion chamber. The corresponding air ratio is lambda > 1.