An adatom is an atom that lies on a crystal surface, and can be thought of as the opposite of a surface vacancy. This term is used in surface chemistry and epitaxy, when describing single atoms lying on surfaces and surface roughness.  The word is a portmanteau of "adsorbed atom".  A single atom, a cluster of atoms, or a molecule or cluster of molecules may all be referred to by the general term "adparticle".  This is often a thermodynamically unfavorable state. However, cases such as graphene may provide counter-examples.