The A term in the van Deemter equation. It is the contribution to plate height from the heterogeneity in axial velocities as a result of the particle size and geometry of the packing, as well as wall effects; A = 2λd_p, where λ is an empirical column constant. Typical values of λ for well-packed columns are 0.8–1.0. Some theories of chromatography indicate a velocity-dependent contribution to the height equivalent to a theoretical plate (HETP) from this process. Also known as eddy diffusion, flow-heterogeneity induced broadening, and the multipath term. See also van Deemter equation.