Peptide
In short A peptide is a chain of two to around fifty amino acids joined by peptide bonds. From around fifty amino acids upwards, the term protein is used.
Research peptides are synthesised chemically, usually by solid-phase synthesis, and supplied as a lyophilised powder. Their identity is described by sequence, molar mass and CAS number; the purity of a lot is on the laboratory's certificate of analysis.
Example: BPC-157 is a pentadecapeptide, a chain of 15 amino acids with a molar mass of 1419.55 g/mol.
The boundary with proteins is a convention, not a law of nature: the usual cut-off is around fifty links. In the UKPeps range, the span runs from three links in the tripeptides to molecules of several thousand grams per mole. A peptide is built from amino acids one link at a time.
Only the sequence determines identity. Two preparations with the same order of links are the same substance, whoever made them. What differs is how cleanly the synthesis ran and what else is in the vial.
A special case in the catalogue is NAD+: a coenzyme made of two nucleotides and therefore not a peptide at all. It is listed because it turns up in the same models of cellular ageing. The price list gives an overview of every substance and vial size.