molecularity
Picture the single, real moment when a chemical change actually happens — the instant bonds break and form. How many particles have to come together in that one event? Sometimes a lone molecule simply falls apart; sometimes two must collide; very rarely three meet at once. Molecularity is the headcount of particles taking part in a single elementary step.
Formally, molecularity is the number of reactant species (atoms, molecules, or ions) that come together in one elementary reaction step: unimolecular for one, bimolecular for two, termolecular for three. It is always a small whole number and is a theoretical property of a single proposed step, defined from the mechanism, not measured directly. Because it describes one real collision event, you can write that step's rate law straight from its molecularity.
Molecularity matters because it is the microscopic story behind the kinetics, and it must be kept distinct from order. Order is an experimental number describing the overall reaction's dependence on concentration; molecularity is the particle count in one mechanistic step. The honest caveat: molecularity is only meaningful for an elementary step, never for an overall multi-step reaction, and termolecular steps are rare because a precise three-body collision is so unlikely.
Ozone breaking down is often modelled with a unimolecular step (one ozone molecule splitting) followed by a bimolecular step (an oxygen atom striking another ozone). The first step involves one particle, the second involves two — those headcounts are their molecularities.
Molecularity counts the particles in one elementary step.
Molecularity is always a whole number (1, 2, rarely 3) and applies only to elementary steps; order is experimental and can be zero, fractional, or negative. Don't equate the two for overall reactions.