In chemistry, functionality is the presence of functional groups in a molecule. A monofunctional molecule possesses one functional group, a bifunctional (or difunctional) two, a trifunctional three, and so forth. In organic chemistry (and other fields of chemistry), a molecule's functionality has a decisive influence on its reactivity.
In polymer chemistry, the functionality of a monomer refers to its number of polymerizable groups, and affects the formation and the degree of crosslinking of polymers.