Peptide: A molecule consisting of 2 or more amino acids. Peptides are smaller than proteins, which are also chains of amino acids. Molecules small enough to be synthesized from the constituent amino acids are, by convention, called peptides rather than proteins. The dividing line is at about 50 amino acids. Depending on the number of amino acids, peptides are called dipeptides, tripeptides, tetrapeptides, and so on.