Therefore any single-chain protein or any chain of amino acids can be called a peptide.
Polypeptide refers to a longer protein.
The words dipeptide, tripeptide and tetrapeptide are in common use to describe amino acid chains of two, three or four amino acids, respectively. For example, the artificial sweetener aspartame is a dipeptide composed of the amino acids aspartate and phenylalanine. Some hormones are called peptide hormones because they are short amino acid chains. For example, the hormone vasopressin, which regulates blood pressure, is a peptide of 9 amino acids.