Classical diffusion is a key concept in fusion power and other fields where a plasma is confined by a magnetic field within a vessel. It considers collisions between ions in the plasma that causes the particles to move to different paths and eventually leave the confinement volume and strike the sides of the vessel.
The rate of diffusion scales with 1/B2, where B is the magnetic field strength, implies that confinement times can be greatly improved with small increases in field strength. In practice, the rates suggested by classical diffusion have not been found in real-world machines, where a host of previously unknown plasma instabilities caused the particles to leave confinement at rates closer to B, not B2, as had been seen in Bohm diffusion.
The failure of classical diffusion to predict real-world plasma behavior led to a period in the 1960s known as "the doldrums" where it appeared a practical fusion reactor would be impossible. Over time, the instabilities were found and addressed, especially in the tokamak. This has led to a deeper understanding of the diffusion process, known as neoclassical transport.