Polymer
Definition
A giant molecule built by linking many small repeating units: plastics, rubber, DNA, and proteins are all polymers.
Technical definition
A polymer is a large molecule composed of many repeating structural units (monomers) joined by covalent bonds. Properties depend on monomer identity, chain length, branching, and cross-linking, spanning synthetic plastics to biological macromolecules.
Example
Polyethylene, the world's most common plastic, is a long chain of thousands of ethene units linked together.
Common confusion
Polymer describes the big molecule; plastic describes a material property. Not all polymers are plastics: starch is a polymer but not a plastic, and plastics contain additives beyond the polymer.