The number of molecules involved in forming the activated complex in a step of a chemical reaction. Reactions are said to be unimolecular, bimolecular, or trimolecular according to whether 1, 2, or 3 molecules are involved.
Chemical Engineering
The number of molecules or ions involved in a chemical reaction. Unlike the order of a reaction, the molecularity must be a whole number. It cannot be fractional or zero.