Anomalous diffusion is a term used to describe a diffusion process with a non-linear relationship to time, in contrast to a typical diffusion process, in which the mean squared displacement (msd), σr2, of a particle is a linear function of time.
Diffusion is often described by a power law[1][2], σr2 ~ Dtα, where D is the diffusion coefficient and t is the elapsed time. In a typical diffusion process, α = 1. If α > 1, the phenomenon is called super-diffusion. Super-diffusion can be the result of active cellular transport processes. If α < 1, the particle undergoes sub-diffusion. Sub diffusion has been proposed as a measure of macromolecular crowding in the cytoplasm.