NMVOC: Non-Methane Volatile Organic Compounds.
It is a generic term for a large variety of chemically different compounds, like for example, benzene, ethanol, formaldehyde, cyclohexane, 1,1,1-trichloroethane. Essentially, NMVOCs are identical to VOCs, but with methane excluded.
NMVOC is also used as a sum parameter for emissions, where all NMVOC emissions are added up per weight into one figure.