Adsorption = accumulation of substance ON a surface (surface phenomenon). Absorption = penetration of substance INTO the bulk (bulk phenomenon). Sorption = both occurring together. Adsorbent = surface on which adsorption occurs (charcoal, silica gel, alumina). Adsorbate = substance being adsorbed (gas or solute). Desorption = release of adsorbate from surface (reverse of adsorption). Adsorption is always exothermic ($delta_H$ < 0) because surface energy decreases. Since adsorption decreases randomness (gas to surface), $delta_S$ < 0. For spontaneous adsorption: $delta_G$ < 0, meaning |T*$delta_S$| < |$delta_H$|.