The reaction-center chlorophyll, or photosystem II primary donor, is a molecular dimer linked with Photosystem II in plants, algae, and cyanobacteria, and is essential for oxygenic photosynthesis.
PS II is a multi-subunit membrane-protein complex participating in oxygenic photosynthesis.
When absorbing a photon, P680 is a collection of pigments that are excitonically connected or function as if they are a single molecule.
It got its name from the wavelength (in nanometers) at which it captures the most light.
It is the electromagnetic spectrum at 680 nm in this case.