A situation in which the freedom of molecules of adsorbate to move about the surface is limited. adsorption is immobile when k.T is small compared to Δ E, the energy barrier separating adjacent sites. The adsorbate has little chance of migrating to neighbouring sites and such adsorption is necessarily localized.
Source:
PAC, 1976, 46, 71. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units - Appendix II. Definitions, Terminology and Symbols in Colloid and Surface Chemistry. Part II: Heterogeneous Catalysis' on page 76 (https://doi.org/10.1351/pac197646010071)