Proteins are synthesized from amino acids, but the process is a condensation reaction which removes a molecule of water from each amino acid. As a result, the amino acid that enters the chain is similar, but not in fact exactly the same as what is after it is part of the chain. The amino acid monomer is now referred to as a "residue", since it is the residue of the polymerization. This picture of cysteine and the cysteine residue should make it clearer.

I'm not going to start a one-man effort to change the world, but it this is something that we should all be aware of, and consider the use of "residue" if appropriate in discussion of polymers and their monomers.
[*]...which completely overlooks that the repeat unit in the final product is actually methylene - CH2
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