In chemistry and biochemistry, a measure of the electron affinity of a particular molecule. This is a somewhat arbitrary measure, as it is measured and reported as a potential difference between the molecule in question and that of the standard hydrogen electrode, which has been pronounced to be 0. Most redox potentials (denoted E0) are tabulated under a set of (also arbitrary) standard conditions: 1 molar HCl (a pH of 0), 1 molar oxidant, 1 molar reductant and 25 degrees Celsius. Since biomolecules fall apart at extremes of pH, biologically important redox potentials are measured and reported at pH 7 (denoted E0'.) These values are for one-electron transfer processes. For multiple electron processes the individual one-electron values are averaged and this average is called a redox midpoint potential.

Redox potentials are measured in volts, and a good way to compare them is to remember that electrons always want to go to the molecule with the highest potential. So, for example if you were trying to figure out if NADPH (with a redox midpoint potential of -320 mV) would give it's two electrons to flavin mononucleotide (-205 mV), you could just by comparing the two values predict that it should happen spontaneously.

The probability of it actually happening depends inversely on how bad you need your experiment to work. But that is meat for another node.