The molecularity of a reaction is defined as the number of reacting molecules which collide simultaneously to bring about a chemical reaction. In other words, the molecularity of an elementary reaction is defined as the number of reactant molecules taking part in the reaction.
For example, consider the reaction
H2 + I2 → 2HI
For a complex reaction the order and molecularity is the same and the order with respect to each reactant is equal to its stoichiometric coefficient in the chemical reactions. In this reaction, the molecularity is two.
Therefore, molecularity is the number of reacting species (molecules, atoms or ions) that collide simultaneously to bring about a chemical reaction is called molecularity of a reaction.