Elementary Step
Definition
A single, simple collision or rearrangement in a reaction mechanism: the individual frames of the reaction's movie.
Technical definition
An elementary step is a single molecular event in a reaction mechanism, classified as unimolecular, bimolecular, or termolecular. Its rate law follows directly from its molecularity: unlike overall reactions, orders DO equal coefficients here.
Example
In one proposed mechanism step, NO + O2 -> NO3 is an elementary bimolecular step with rate = k[NO][O2].
Common confusion
Termolecular elementary steps, three molecules colliding at once, are extremely rare. If a proposed mechanism relies on one, it is probably wrong.