A particle, either an atom or a molecule, that has been adsorbed onto a surface (called the substrate). Basically, that means the particle has stuck to the surface. This is different from absorption, where a particle enters the bulk of the solid.
Adsorption is achieved in two major ways: physisorption and chemisorption. In physisorption, there is a weak interaction between the surface and the adsorbate, usually the van der Waals force. In chemisorption, something similar to a chemical bond forms between a surface atom and the adsorbate.
Also, molecular adsorption can either be associative, where a molecule is adsorbed as a whole, or dissociative, where the molecule is broken into parts when it is adsorbed. Usually physisorption is associative and chemisorption is dissociative.
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surface science gets more complicated from here...