Peptides are polymers of amino acids. Two amino acid molecules can be covalently joined through a substituted amide linkage, termed a peptide bond, to yield a dipeptide. Such linkage is formed by removal of the elements of water from the a-carboxyl group of one amino acid and the a-amino group of another. Peptide-bond formation is an example of a condensation reaction, a common class of reaction in living cells.

The amino acid units in a peptide are often called residues (each has lost a hydrogen atom from its amino group and a hydroxyl moiety from its carboxyl group). The amino acid residue at that end of a peptide having a free a-amino group is the amino-terminal (or N-terminal) residue; the residue at the other end, which has a free carboxyl group, is the carboxyl-terminal (C-terminal) residue. By convention, short peptides are named from the sequence of their constituent amino acids, beginning at the left with the amino-terminal residue and proceeding toward the carboxyl terminus at the right.