(1) Of ions. The spontaneous migration of ions from a more concentrated solution into a more dilute solution. Compare osmosis.
(2) Of thermal neutrons. A net movement or flow of thermal neutrons from regions of higher neutron concentration to regions of lower concentrations. The neutrons are moving with velocities corresponding to their energies in more or less random directions (due to repeated scattering interactions). Thus, there is a tendency for more neutrons to leave the higher-concentration zone and for fewer to enter it.
An example case is when the neutron population in the borehole is much lower than that in the formation because the neutrons are rapidly captured by the abundant chlorine nuclei in the very saline borehole fluid. More neutrons will diffuse from formation to borehole than from borehole to formation, and there will be a net flow of neutrons from formation to borehole.
There is generally a net diffusion flow away from the source (i.e., toward zones of lower neutron concentration) and a tendency for net flow from a zone of smaller capture cross section to one of larger capture cross section.