The adhesion of a substance to the surface of a solid or liquid. Adsorption is often used to extract pollutants by causing them to attach to such adsorbents as activated carbon or silica gel. Hydrophobic (water-repulsing) adsorbents are used to extract oil from waterways in oil spills.
The adherence of dissolved, colloidal, or finely divided solids to the surface of solid bodies when they are brought into contact.

Source: Handbook of water and wastewater treatment plant operations