The Collaborative International Dictionary of English v.0.48:

adsorption \adsorption\ n.
   1. 1 the process by which molecules of a substance form a
      thin film on the surface of a solid. Distinguished from
      absorption, in which the foreign substance penetrates
      the body of the absorbing material.

   Syn: surface assimilation
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

adsorption
    n 1: the accumulation of molecules of a gas to form a thin film
         on the surface of a solid [syn: adsorption, surface
         assimilation]