The coefficient for molecular diffusion along the flow path. It is represented as the B term in the van Deemter equation. The greater the diffusion coefficient of the component in the mobile phase, the more the narrow band of separated component diffuses into the surrounding mobile phase before going to the detector. This results in band broadening. As linear velocity increases, the effect of axial diffusion on column efficiency decreases. The van Deemter equation reflects this effect by dividing B by u.