(Science: radiobiology) diffusion in most plasma devices, particularly tokamaks, is higher than what one would predict from understood causes. The observed, typical diffusion is referred to as anomalous because it has not yet been explained.
anomalous diffusion includes all diffusion which is not due to collisions and geometric effects. While such effects were not understood when the term was coined, and most still are not, diffusion due to well-understood wave phenomena is still 'anomalous'. Classical diffusion and Neo-classical diffusion are the two well-understood diffusion theories, although neither is adequate to fully explain the observed anomalous diffusion.