Monomer
Definition
The small molecule that links up with others to build a polymer: like a single Lego brick in a huge structure.
Technical definition
A monomer is a small molecule capable of bonding to others to form a polymer chain, typically through addition polymerization (opening a double bond) or condensation polymerization (eliminating a small molecule like water).
Example
Ethene (C2H4) is the monomer of polyethylene. Glucose is the monomer of starch and cellulose.
Common confusion
A monomer is not a piece broken off the polymer. Polymerization transforms monomers (double bonds open, small molecules may leave), so the repeating unit in the chain differs slightly from the free monomer.