HPLC adsorbents (often called "stationary phases") are the materials which after being packed in the column actually retain (adsorb) analytes and produce a separation. "Stationary phase" stands for that part of the chromatographic system which is in equilibrium with the mobile phase. In general, HPLC system consists of two phases: liquid (mobile) and solid (stationary). In a few cases analyte molecules can penetrate into the stationary phase: liquid-liquid chromatography, and chromatography on the soft gels (agarose). In all others, (the great majority of HPLC applications) solid stationary phase is rigid and impermeable for the analyte molecules. Only the surface of the stationary phase actually participates in the chromatographic retention process. So, chromatography is the dynamic interface (adsorption) phenomena. We have to treat a stationary phases as an adsorbent and consider its surface properties.
The dominating type of HPLC adsorbents are silica gel.