Diffuson
The diffuson is a mathematical object, which often appears in the theory of disordered electronic systems (a part of condensed matter physics).
In a disordered system, the motion of an electron is not ballistic, but diffusive: i.e., the electron does not move along a straight line, but experiences a series of random scatterings off of impurities. This random motion (diffusion) is described by a differential equation, known as the diffusion equation. The diffuson is the Green's function of the diffusion equation.