Reaction mechanism, or reaction pathway, describes the successive steps at the molecular level that take place in a chemical reaction. In each step, molecular bonds are either created or broken. Reaction mechanisms are postulated, and then either supported or disproved but the overarching conditions for defining a reaction mechanism/reaction pathway is that all of the discrete steps together make sense with regard to the overall reaction equation, and that the rate law for the slowest step complies with the empirically measured overall rate.
The individual steps that make up most reaction mechanisms are described as unimolecular or bimolecular. A unimolecular mechanism occurs when a molecule either dissociates to form two species or undergoes internal rearrangement. The rate equation for a unimolecular reaction is described as first order since it only depends on the concentration of one reactant and is written as d[A]/dt= - k[A], where k is the rate constant. A bimolecular reaction occurs when two molecules combine and the rate equation is d[A]/dt = - k[A][B] (second order) or second order in one of the molecular species, d[A]/dt = - k[A]2
