In automotive engineering lambda, the 11th letter of the Greek alphabet, represents the air to fuel ratio the engine is operating at.
The chemically perfect or stoichiometric air to fuel ratio is about 14.7 parts air to one part fuel (by mass).
Lambda = 1 means the engine is running at this 'perfect' air to fuel ratio.
Lamda > 1 means the engine has excess air and is running on a lean mixture.
Lambda < 1 means the engine has excess fuel and is running on a rich mixture.
Modern engines use a lambda sensor to measure the air to fuel ratio.