Adsorption shall be distinguished from absorption, where a substance diffuses in a liquid or solid and forms a solution or gel. The term "sorption" combines both notions. The substance on whose surface adsorption occurs is called the adsorbent, and the absorbed substance - the adsorbate. Depending on the nature of the interaction between the adsorbate molecules and the adsorbent the process of adsorption is subdivided into physical adsorption (weak interaction) and chemisorption (strong interaction). There is no clear boundary between physical adsorption and chemisorption; the binding energy between the adsorbate and the adsorbent, equal to 0.5 eV per atom or molecule, has been adopted as a boundary value.

The reverse of adsorption is desorption. The condition when the rates of adsorption and desorption are equal is called the adsorption equilibrium. In a state of equilibrium, the number of adsorbed molecules remains constant so long as the external conditions-pressure, temperature, composition of the system - remain unaltered.

In practice, adsorption is widely used for the concentration of substances and gas and liquid purification. Adsorption methods of analysis are used to determine the specific surface area of solids and to evaluate the size of applied particles (including nano scale) on the surface of a carrier, etc.