The propensity of part of a flowing fluid to accelerate through a system such as a pipe, vessel, or reactor, and to reach the exit point having effectively short-circuited the rest of the fluid. Forward mixing leads to reduced reactor efficiencies where some of the reactants that are expected to undergo a chemical reaction pass through unreacted. It is therefore a form of departure from ideality where the flow of fluids leave before the expected time. Conversely, some reactants will reside for longer periods, known as back-mixing.