Refers to the B term (second term) of the van Deemter equation. Also called longitudinal or axial diffusion term. It dominates band broadening only at very low flow rates below the minimum plate height at which the diffusion of individual solutes can occur in a longitudinal (lengthwise) direction on the column. The contribution to the B term arises from diffusion in the mobile phase and is 2γD_M, where γ is the obstruction factor (typically 0.6–0.8) and D_M is the diffusion coefficient. See also van Deemter equation.