A concept used to characterize the macro-mixing in a continuous flow system such as a reactor. It is expressed as the fraction of a material remaining in the system as a function of time given either as a cumulative distribution or as the derivative of the cumulative curve. The RTD can be determined by adding either a step increase in a tracer substance or an instantaneous pulse of a tracer into the flow system, (p. 326) and measuring the response at the other end or output. The RTD information is useful to determine the performance of a reactor throughput.