Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid is dissolved by or permeates a liquid or solid. While adsorption does often precede absorption, which involves the transfer of the absorbate into the volume of the absorbent material, alternatively, adsorption is distinctly a surface phenomenon, wherein the adsorbate does not penetrate through the material surface and into the bulk of the adsorbent. The term sorption encompasses both adsorption and absorption, and desorption is the reverse of sorption.

Paul Hugh Emmett was an American chemist best known for his pioneering work in the field of catalysis and for his work on the Manhattan Project during World War II. He spearheaded the research to separate isotopes of uranium and to develop a corrosive uranium gas. Emmett also made significant contributions to BET Theory which explains the relationship between surface area and gas adsorption. He served on the faculty of Johns Hopkins University for 23 years throughout his scientific career.