Definition
Residue
A residue is a monomer unit as it appears once incorporated into a polymer chain. The term marks the chemical fact that joining monomers into a chain discards a small fragment — typically a water molecule — so the unit inside the chain is not identical to the free monomer.
Formally, if a polymer is a word over a monomer alphabet , then each is a residue: a letter of the alphabet in its incorporated, bonded form. The corresponding free monomer and the residue differ by the atoms lost to the condensation reaction that formed the bond.