Convective flow through a porous membrane that carries chromatographic ligands, so that intraparticle diffusion is replaced by through-pore convection, defines membrane chromatography. Ion-exchange, affinity, or hydrophobic groups are grafted onto the membrane; the feedstock is forced through the pores, and binding occurs on the pore walls with short residence times and low pressure drop ^[1]. The format is therefore a chromatographic adsorber whose geometry is a stack or spiral of membranes rather than a packed bed of porous beads. Conceptual boundaries exclude tangential-flow filtration without ligands, packed-bed chromatography, and immobilized-artificial-membrane (IAM) chromatography, in which membrane lipids are coated on silica particles to measure partition coefficients rather than to process a convective membrane stack ^[2]. Essential features are a ligand-functionalized porous membrane, predominantly convective mass transport, and a bind-and-elute or flow-through operating mode ^[3]. Large-scale antibody processes use quaternary-ammonium membranes as a flow-through anion-exchange step, which remains membrane chromatography by geometry and transport, not a new retention chemistry ^[4].