Diffusion damping (aka Silk damping) is a process presumed to occur during recombination, the damping being a reduction in the density of the denser regions, i.e., reducing CMB anisotropies, i.e., those observed in the CMB. The mechanism is increased diffusion of photons as the plasma grew more transparent (as their mean free path grows, the resulting energy diffusion gradually increases), and diffusion damping model calculates the slight tendency (on average) for photons to move away from dense regions. Plasma up to recombination included many photons, enough to be in equilibrium with the baryons, i.e., as a group, photons were being created as much as they were absorptioned and gaining energy as much as they were losing it. The baryon acoustic oscillations constituted density waves at that time, and diffusion damping tended to reduce them. A result in diffusion damping is a reduction in the higher coefficient of the CMB anisotropy power spectrum. The resulting tail of the power spectrum is termed the damping tail.