What is reaction centre ? Why these are called P700 and P680?

A photosynthetic reaction centre is a complex of several proteins, pigments and other co-factors that together execute the primary energy conversion reactions of photosynthesis. The single chlorophyll a molecule forms the reaction centre. The reaction centre is different in both the photosystems. In PSI the reaction centre chlorophyll a has an absorption peak at 700nm, hence is called P700, while in PSII it has absorption maxima at 680 nm and is called P680.