FCS stands for fluorescence correlations spectroscopy (FCS). FCS is a method which relies on fluctuations on the recorded signals to characterize molecular interaction such as binding and unbinding, chemical reaction kinetics, diffusion of fluorescent molecules. 1,2

Elson EL, Magde D. Fluorescence Correlation Spectroscopy. I. Conceptual Basis and Theory. Biopolymers. 1974;13:1-27.
Magde D, Elson EL, Webb WW. Thermodynamic fluctuations in a reacting system – measurement by fluorescence correlation spectroscopy. Physical Review Letters. 1972;29:705-708.