An expanded bed is a fluidized chromatographic packing whose particles are lifted by upward liquid flow until a stable, classified bed is formed, and this hydrodynamic state defines expanded-bed chromatography. The interstitial volume is large enough that cells, debris, and other particulates in a crude feedstock pass through the bed, while the target solute binds to ligands on the adsorbent ^[1]. When flow is stopped or reversed, the bed settles and the captured solute is eluted as in a packed column. The mode is therefore a primary capture operation that combines clarification and adsorption in one unit, distinct from a packed-bed column that would clog on unclarified feed and from a true well-mixed fluidized bed that lacks axial classification ^[2]. Essential features are a density- and size-distributed adsorbent, an upward liquid velocity between the minimum expansion velocity and the particle terminal velocity, and a ligand chemistry (ion exchange, hydrophobic, affinity, or mixed mode) chosen for the target ^[3]. Conceptual boundaries exclude conventional packed-bed chromatography and continuous annular beds that do not rely on stable expansion.