Monomers and polymers At Wikipedia a monomer is defined to be


  a molecule that may bind chemically to other molecules to form a
  polymer.


A polymer is defined to be


  a molecule composed of many monomers.


These definitions are obviously circular.

Are there non-circular definitions of 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.