A type of idealized chemical reactor vessel used to contain a chemical reaction in which liquid reactants continuously flow into the reactor and products continuously flow out such that there is no accumulation within the reactor. By assuming perfect mixing of the reactants within the reactor by using stirring, the composition of the reaction does not vary with either position or time, and the output composition of the material is therefore assumed to be the same at the composition at all points within the reactor. It is also known as a back-mix reactor.