Monomers and polymers
The term "monomer" only has meaning in the context of discussing polymerization.

A polymer is a molecular chain where the links of the chain are repeated identical or a least similar units.  Examples where the units are idential would be polypropylene, polyvinylchloride and polysiloxane.  Example where the units are similar but not necessarily identical would be DNA and proteins.  

A monomer is a molecule that can react to form a single unit of a polymer.  

For example, propylene is a monomer of poylpropylene and amino acids are monomers of a protein.