The active solid, stationary liquid plus solid support, or swollen gel put in the column. The term packing refers to the conditions existing before the chromatographic run is started (i.e. to the material introduced into the column), whereas the stationary phase refers to the conditions during the run.
See also: totally porous packing, pellicular packing
Orange Book,2nd ed., p. 97
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 831 (https://doi.org/10.1351/pac199365040819)