functionality, f

https://dx.doi.org/10.1351/goldbook.FT07505
Number of covalent bonds that a monomer molecule or monomeric unit in a macromolecule or oligomer molecule can form with other reactants.
Notes:
  1. There are no monofunctional monomers.
  2. If f = 2, a linear chain macromolecule or a macrocycle can be formed.
  3. If f > 2, a branch point can be formed leading to a branched macromolecule, a network or a micronetwork.
  4. Ethene and ethylene glycol are examples of difunctional monomers, glycerol is an example of a trifunctional monomer, and divinylbenzene and pentaerythritol are examples of tetrafunctional monomers.
Source:
PAC, 2007, 79, 1801 (Definitions of terms relating to the structure and processing of sols, gels, networks, and inorganic-organic hybrid materials (IUPAC Recommendations 2007)) on page 1805