Stability in a peptide-chemistry context refers to the retention of chemical integrity, purity and biological activity of a peptide across time under a defined set of storage conditions (temperature, humidity, light, container material, aqueous vs lyophilised state).
The principal degradation pathways relevant to peptides are proteolytic and non-enzymatic hydrolysis of peptide bonds, oxidation of methionine and cysteine residues, deamidation of asparagine and glutamine, aggregation, and adsorption to container surfaces. Lyophilisation dramatically slows most of these processes, which is why research peptides are typically supplied as freeze-dried powders.
Stability data determine expiry, storage temperature and handling recommendations for every research peptide; it is a core Certificate of Analysis parameter.