Monomer

Monomer: A substance capable of forming covalent bonds with a sequence of additional like or unlike molecules under the conditions of the relevant polymer-forming reaction used for the particular process. Examples: propylene, terephthalic acid, caprolactam. An unreacted monomer is an intended component in a polymeric material. A residual monomer is an unintended impurity in a polymer.
[Definitions from REACH and GreenScreen v1.4 ]