In chemistry, adsorption refers to the accumulation of molecules of a gas or liquid on the surface of another substance; however, the accumulating molecules do not actually penetrate the substance they're on.
In virology, adsorption refers to the adhesion of a substance to an organic particle in a solution. For instance, it could be used to describe the adhesion of a virus to a cell.
Contrast either of the above with absorption.
From the science dictionary at http://biotech.icmb.utexas.edu/