In monolayer adsorption all the adsorbed molecules are in contact with the surface layer of the adsorbent.
In multilayer adsorption the adsorption space accommodates more than one layer of molecules and not all adsorbed molecules are in contact with the surface layer of the adsorbent.
The monolayer capacity is defined, for chemisorption, as the amount of adsorbate which is needed to occupy all adsorption sites as determined by the structure of the adsorbent and by the chemical nature of the adsorptive; and, for physisorption, as the amount needed to cover the surface with a complete monolayer of molecules in close-packed array, the kind of close-packing having to be stated explicitly when necessary. Quantities relating to monolayer capacity may be denoted by subscript 
.
The surface coverage (
) for both monolayer and multilayer adsorption is defined as the ratio of the amount of adsorbed substance (see §§1.1.8 to 11) to the monolayer capacity.
The area occupied by a molecule in a complete monolayer is denoted by 
; for example, for nitrogen molecules
).
Micropore filling is the process in which molecules are adsorbed in the adsorption space within micropores7.
The micropore volume is conventionally measured by the volume of the adsorbed material, which completely fills the micropores, expressed in terms of bulk liquid at atmospheric pressure and at the temperature of measurement8.
In certain cases (e.g. porous crystals) the micropore volume can be determined from structural data.
Capillary condensation is said to occur when, in porous solids, multilayer adsorption from a vapour proceeds to the point at which pore spaces are filled with liquid separated from the gas phase by menisci9.
The concept of capillary condensation loses its sense when the dimensions of the pores are so small that the term meniscus ceases to have a physical significance. Capillary condensation is often accompanied by hysteresis.