Molecularity in chemistry is the number of colliding molecular entities that are involved in a single reaction step.
* A reaction step involving one molecular entity is called unimolecular.
* A reaction step involving two molecular entities is called bimolecular.
* A reaction step involving three molecular entities is called trimolecular or termolecular.
In general, reaction steps involving more than three molecular entities do not occur, because is statistically improbable in terms of Maxwell distribution to find such a transition state.