Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:
In atmospheric chemistry and air pollution and related fields, nitrogen oxides refers specifically to NOx (NO and NO2).
Only the first three of these compounds can be isolated at room temperature. N2O3, N2O4, and N2O5 all decompose rapidly at room temperature. Nitrate radical is very reactive.
N2O is stable and rather unreactive at room temperature, while NO and NO2 are quite reactive but nevertheless quite stable when isolated.