Complex mechanisms: from elementary steps to the rate law

An elementary step happens in a single event at the molecular scale. For it — and for it alone — each reactant's partial order is its coefficient: unimolecular $v = k[\ce{A}]$, bimolecular $v = k[\ce{A}][\ce{B}]$. Molecularity $3$ is rare: recombination $\ce{A + B + M -> AB + M}$, where the third body M carries away the excess energy.