Repeat unit
A repeat unit or repeating unit , or mer, is a part of a polymer whose repetition would produce the complete polymer chain (except for the end groups) by linking the repeat units together successively along the chain, like the beads of a necklace.
constitutional unit: An atom or group of atoms (with pendant atoms or groups, if any) comprising a part of the essential structure of a macromolecule, an oligomer molecule, a block or a chain.
constitutional repeating unit: The smallest constitutional unit the repetition of which constitutes a regular macromolecule, a regular oligomer molecule, a regular block or a regular chain.
A repeat unit is sometimes called a mer (or mer unit) in polymer chemistry. "Mer" originates from the Greek word meros, which means "a part". The word polymer derives its meaning from this, which means "many mers". The mer is not the same thing as a monomer—a mer is a repeating unit within a larger molecule, whereas a monomer is an actual molecule that exists independently, either prior to polymerization or after decomposition.