Reverse electron flow (also known as reverse electron transport) is a mechanism in microbial metabolism.  Chemolithotrophs using an electron donor with a higher redox potential than NAD(P)/NAD(P)H, such as nitrite or sulfur compounds, must use energy to reduce NAD(P).  This energy is supplied by consuming proton motive force to drive electrons in a reverse direction through an electron transport chain and is thus the reverse process as forward electron transport.  In some cases, the energy consumed in reverse electron transport is five times greater than energy gained from the forward process.  Autotrophs can use this process to supply reducing power for inorganic carbon fixation.
Reverse electron transfer (RET) is the process that can occur in respiring mitochondria, when a small fraction of electrons from reduced ubiquinol is driven upstream by the membrane potential towards mitochondrial complex I. This results in reduction of oxidized pyridine nucleotide (NAD or NADP). This is a reversal of the exergonic reaction of forward electron transfer in the mitochondrial complex I when electrons travel from NADH to ubiquinone.