In organic chemistry, functionality is often used as a synonym for functional group. For example, a hydroxyl group can also be called a HO-function.
Functionalisation means the introduction of functional groups, for example
* the functionalisation of a surface (e.g. silanization for the specific modification of the adhesion of a surface)
* the functionalization of nanoparticles of a metal or metal oxide to stabilize such nanoparticles or
* the so-called  C-H functionalization, which means the substitution of a C-H bond by a functional group, bonded at the same carbon atom