What a peptide is

A peptide is a short chain of amino acids linked by amide bonds. The convention is loose but useful: up to about 20 residues is a peptide, 20–50 is a polypeptide, and beyond that the chain folds into a protein. The distinction matters commercially because short chains can be made by solid-phase synthesis to a defined sequence, while longer ones are usually produced by hydrolysis or recombinant routes and sold as distributions rather than single molecules.

For a buyer, the practical consequence is that “purity” means something different in each case. A synthesised peptide has a target sequence and a measurable impurity profile. A hydrolysate has a molecular-weight distribution and a protein content, and no single CAS number that describes it.