1. n. 
The movement of ions or molecules from regions of high concentration to low concentration within a solution.
2. n. 
The conduction of heat by the movement of molecules.
3. n. 
The process by which particles move over time within a material due to their kinetic motion. The term is most commonly used in pulsed neutron capture logging and in nuclear magnetic resonance (NMR) logging. In a pulsed neutron capture log, the term refers to the spread of neutrons away from the neutron generator. In NMR logging, diffusion refers to the movement of gas, oil, or water molecules within the pore space.