Peptide
In plain language
A short chain of amino acids — smaller than a protein — that the body uses as a signaling molecule.
Technical definition
A molecule composed of two or more amino acids linked by peptide bonds, generally shorter than proteins; many act as hormones or signaling agents.
Peptides sit between individual amino acids and full-length proteins in size. Because they are broken down in the gut, most therapeutic and research peptides are given by injection rather than swallowed.