Please tell me what NOx means

Nitrous Oxide

Smoothy-not quite. N20 is Nitrous Oxide. NOx refers to oxides of Nitrogen and since Nitrogen has different oxides, the x stands for different oxygen atoms. NO is Nitric Oxide, NO2 is Nitrogen Dioxide. These are both commonly found in exhaust gasses. Nitrous Oxide, N2O, at elevated temperatures, is a powerful oxidizer similar to molecular oxygen. That's how cars boost horsepower by injecting N2O into the intake manifold. It is converted to NO.

True... but it is brandished around in the racing world as that (mostly by the boy racer types)... even if its techincally improper.

I've heard it called Nos too, but like you said... the correct term is N2O.

If he's looking at the resulats of an emmissions test printout (or a Chenistry class)... most deinately Oxides of Nitrogen (not the same as Nitros Oxide).