Molecularity is the number of molecules involved in forming the product. This follows from the balanced (stoichiometric) equation describing the reaction. For example,
N2O5 —» 2NO2 + 1/2 O2 is a slow unimolecular reaction and 1/2 O2 + 1/2 O2 —» O2 is a fast bimolecular reaction.
