A peptide is a molecule composed of two or more amino-acid residues covalently linked by peptide (amide) bonds. By convention, chains of up to roughly 50 residues are described as peptides; longer chains that adopt a stable tertiary fold are described as proteins, though the boundary is not sharp.
Peptides are typically written from N-terminus (free amine) to C-terminus (free carboxyl) using the one- or three-letter amino-acid code. Their primary structure — sequence, length, and any post-translational or synthetic modifications (acetylation, amidation, PEGylation, lipidation, cyclisation) — determines physicochemical properties such as solubility, charge, stability and receptor affinity.
Peptide is the base term underneath every compound in this catalogue. Receptor ligands, mimetics and reference standards are all classes of peptides differentiated by sequence and modification strategy.