Molecularity: Unimolecular and Bimolecular Steps
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An elementary step's molecularity is its number of particles: a unimolecular step involves one, a bimolecular step involves two.
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Cyclobutane's decomposition to ethylene is a single unimolecular step; in ozone's mechanism, O3 reacting with O to form two O2 is bimolecular.
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Molecularity counts particles in one elementary step — it describes an overall reaction only for a single-step reaction.