Purity
In short The proportion of the intended substance in the sample analysed, expressed as a percentage.
Purity answers the question: of everything in this sample, how much is the peptide we meant to make? The rest is usually closely related material from synthesis, such as sequences missing one amino acid, plus residual salts and water.
Our written specification for peptides is at least 98%. The specification is what we require; the value actually measured for a lot is on that lot's certificate. The two figures should not be confused, and a page that gives only the specification tells you nothing about a particular vial.
How purity is measured matters. HPLC separates components and expresses the main peak as a share of all detected peaks; peptide screening reports results differently. Our laboratories use different methods, so we always show the method next to the figure, and we do not average results across methods.
Some certificates confirm identity without stating a purity figure. We publish those as they are rather than inventing a number. In the certificate library they appear as "not stated".