Lambda is the alternative scale for Air Fuel ratio. 14.7 = 1.0 lambda, lambda above 1 = lean, below 1 = rich. This can only be checked with a wide range AFR instrument as cars O2 sensors are narrow band designed to basically trigger digitally trigger the ECU with on/off info, i.e. above stoichiometric 14.7 or below.
I have a maybe-important, maybe not (depending on what you're doing) quibble with this statement.
Lambda (λ) is, in fuel chemistry, the stoichiometric ratio of fuel to air: the proportion at which there is exactly
enough oxygen available to completely combust the fuel present. The energy density of fuel varies significantly, even at the pump by the side of the road. If you have a fuel with no ethanol in it with an average gasoline density, you'll have an AFR of about 14.7 at lambda. If you're burning E10 (which almost all high test fuel in the US is), then lambda is around 14.13. If you're burning E100, lambda is at an AFR of about 9.
The EGO sensor on the car is actually reporting the presence or absence of free (excess) oxygen in the exhaust. It's a binary yes/no answer: are we above lambda or at/below lambda right now? Accordingly, the ECU constantly dithers the amount of fuel injected to stay a known distance from lambda in one direction or the other.
The ECU isn't concerned with AFR at all. It worries about being out of reasonable bounds of fuel injected per intake stroke given the operating conditions of the moment (which might indicate a component failure), and it is constantly hunting for lambda. That's pretty much it.
Because of the variability and limited meaning to modern systems, it's probably a good idea to stop thinking about AFR at all. It makes sense for carburetors, but not so much for EFI with a lambda sensor.