A reaction the course of which is proportional to the concentration of the single stuff undergoing change; radioactive decay is a first-order process, defined by the equation -(dN/dt)=kN, where N is the number of atoms subject to decay (reaction), t is time, and k is the first-order decay (reaction) constant, i.e., the fraction of all atoms decaying for unit of time.
See: decay constant, order.