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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?
Let's have a look at what IUPAC say; they would be a reliable source to get definitions of chemical terminology from:
"A substance composed of monomer molecules."
"A molecule which can undergo polymerization thereby contributing constitutional units to the essential structure of a macromolecule. "
"A substance composed of macromolecules."
"A molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass. "
These definitions are consistent with the latest IUPAC Compendium of Polymer Terminology and Nomenclature, IUPAC Recommendations 2008
Note that, historically, the definition of a polymer has changed somewhat. Up until about 80 years ago, polymers were seen as molecules comprised of similar empirical formula - benzene, for example, was considered a polymer of acetylene. It was Staudinger's work in the 1920's (contributing to his Nobel Prize) that defined polymers as macromolecules.
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.
