A molecule containing at least two amino acid groups bound by a peptide bond, i.e. the CO-NH2 bond obtained from dehydrating condensation between the NH2 group and the carboxylic group of neighbouring amino acids.
The terms dipeptides and tripeptides are used when the chain only contains two or three amino acid groups the term oligopeptide refers to a short amino acid chain and a polymer contains only one to two tens of groups. The longest amino acid chains are called polypeptides. The term protein is used for these polypeptide chains. A protein may be composed of several chains bound by disulphide bridges. Two examples are insulin that contains 53 amino acid groups and haemoglobin that contains more than 500.
Some peptides act as hormones (pituitary) or as neurotransmitters.