A polymer is a material whose molecules contain long chains built from repeating chemical units.

A repeat unit is the structural motif that recurs along the chain. If a chain contains $N$ repeat units, $N$ is its degree of polymerization.

For example, polyethylene can be represented schematically as

$$[-\mathrm{CH_2-CH_2}-]_N.$$

The covalent bonds along the backbone are strong, while interactions between neighboring chains can be much weaker. This contrast is central to polymer behavior: chains can be individually robust yet rearrange, slide or rotate relative to one another.

A real polymer sample usually contains chains of many different lengths rather than one identical $N$. Increasing average chain length generally increases entanglement and can strongly change melt flow, toughness and processability even though the repeat-unit chemistry is unchanged.

Polymer properties therefore depend not only on which atoms are present but also on how many repeat units are connected into each molecule and how those long molecules are organized.