Step 1: Read the question carefully. We need the name for the number of molecules taking part in a single reaction step.
Step 2: Learn the term molecularity. Molecularity is the count of molecules that come together at the same time in one elementary reaction.
Step 3: Look at simple examples. One molecule reacting: \[ A \rightarrow \text{Products} \] this is unimolecular. Two molecules reacting: \[ A + B \rightarrow \text{Products} \] this is bimolecular.
Step 4: Compare with order of reaction. Order of reaction is found by experiment and can be zero or a fraction. Molecularity is found from the equation and is always a whole number.
Step 5: Rule out the other choices. Unimolecular is only one special case, not the general name. Rate of reaction is the speed, not a count. So those do not fit.
Step 6: Pick the correct word. The number of molecules in a reaction is called molecularity. \[ \boxed{\text{Molecularity}} \]