Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. The substance being adsorbed is the adsorbate, while the material providing the surface is the adsorbent. This process differs from absorption, in which a fluid (the absorbate) is dissolved by or permeates a liquid or solid (the absorbent). Adsorption is a surface phenomenon and does not penetrate through the surface to the bulk of the adsorbent, while absorption involves the whole volume of the material, although adsorption does often precede absorption. The term sorption encompasses both processes, while desorption is the reverse of it.