The reaction center chlorophyll (or the primary electron donor) of photosystem II that is most reactive and best in absorbing light at wavelength of 680 nm.
P680 is a group of pigments that are excitonically coupled or that act as if the pigments are a single molecule when they absorb a photon. It derived its name after the wavelength (in nanometers) at which it is best in capturing. In this case, it is the 680 nm of the electromagnetic spectrum.
P680 is the strongest biological oxidizing agent known at present.