An entity in which there is a host structure, usually a crystal, which can trap an atom, ion, or small molecule. The trapped species can be found in tunnels, interlayer spacings, or cages of the host species, with the inclusion compounds being one-dimensional, two-dimensional, or three-dimensional respectively. A two-dimensional compound is called an intercalation compound, whereas a three-dimensional compound is called a clathrate. Inclusion compounds are not genuine compounds, as the bonds between the host and the trapped species are weak bonds, such as the van der Waals’ force.