What is Elementary step, molecularity, mechanism?

An elementary step is a reaction that takes place in a single molecular event, as it is written, without any intermediate. Its molecularity is the number of particles that react in that event: 1 (unimolecular, a decomposition or rearrangement), 2 (bimolecular, a collision), rarely 3. A reaction mechanism is the set of elementary steps whose sum is the overall reaction. A species formed in one step and consumed in a later one, absent from the overall equation, is a reaction intermediate.