--- title: "Peptide Terminology: A Plain Glossary" canonical_url: https://peptidelibrary.app/blog/peptide-terminology-glossary description: "Peptide listings and research summaries assume a vocabulary. These are the terms that come up constantly, in plain language, with links to where each matters." updated: 2026-09-08 publisher: Peptide Library notice: "Research and educational use only. Not medical advice." --- # Peptide Terminology: A Plain Glossary Peptide listings and research summaries assume a vocabulary. These are the terms that come up constantly, in plain language, with links to where each matters. **Peptide listings and research summaries assume a vocabulary that nobody explains.** These are the terms that come up most, in plain language, with links to where each one actually matters. ## What the molecules are - **Peptide. **A short chain of amino acids. Longer chains become proteins; the boundary is conventional rather than precise, usually around fifty residues. - **Analogue. **A modified version of a natural molecule, altered to change stability, duration, or receptor selectivity. Semaglutide is a GLP-1 analogue. - **Fragment. **A portion of a larger molecule, isolated in the hope of keeping one activity and dropping others. [AOD-9604](https://peptidelibrary.app/blog/aod-9604) and [TB-500](https://peptidelibrary.app/blog/tb-500-dosage) are both fragments. - **Blend. **Multiple peptides in one vial at a fixed ratio, such as [GLOW and KLOW](https://peptidelibrary.app/blog/klow-peptide-blend). ## What they do - **Agonist. **Binds a receptor and activates it. A dual agonist activates two receptor types — tirzepatide at GIP and GLP-1. - **Antagonist. **Binds a receptor and blocks it rather than activating it. - **Secretagogue. **Prompts the body to release something it already makes rather than supplying it. [Ipamorelin](https://peptidelibrary.app/blog/ipamorelin-dosage) is a growth hormone secretagogue — the distinction matters because the ceiling stays the body’s. - **Selective. **Acts on one target without triggering related ones. Ipamorelin’s selectivity is its defining claim. - **Desensitisation. **Receptors becoming less responsive to continued stimulation. It can reverse an effect entirely — see [gonadorelin](https://peptidelibrary.app/blog/gonadorelin). ## Handling and formulation - **Lyophilised. **Freeze-dried to a stable powder. The form nearly all research peptides ship in. - **Reconstitution. **Adding liquid to dissolve that powder. The volume sets concentration, not the amount of peptide — see the [bacteriostatic water guide](https://peptidelibrary.app/blog/bacteriostatic-water-for-peptides). - **Bacteriostatic water. **Sterile water with 0.9% benzyl alcohol, allowing a vial to be re-entered for up to 28 days. - **Beyond-use date. **When a vial should be discarded after first puncture — distinct from the printed expiry. - **Subcutaneous. **Into the fat layer beneath the skin. See [injection sites](https://peptidelibrary.app/blog/subcutaneous-injection-sites). ## Measurement - **mg/mL. **Concentration — milligrams of peptide per millilitre of solution. The number that turns a dose into a syringe reading. - **Unit (syringe). **A volume mark. It says nothing about drug content — see the [syringe guide](https://peptidelibrary.app/blog/insulin-syringe-units-to-ml). - **IU. **International unit — a measure of biological activity, not mass, and not interchangeable with syringe units. Relevant for [oxytocin](https://peptidelibrary.app/blog/oxytocin-peptide). - **mcg. **Microgram, one thousandth of a milligram. Most research peptides are discussed at this scale. ## Quality and testing - **COA. **Certificate of analysis — a lab report for one production batch. See [how to read one](https://peptidelibrary.app/blog/how-to-read-a-peptide-coa). - **HPLC purity. **The proportion of peptide material that is the target compound. Typically 95–99%. - **Net peptide content. **The proportion of the vial’s mass that is actually peptide rather than counter-ions and water. Usually 70–85%, and usually not reported. - **Mass spectrometry. **Confirms molecular weight — answers whether it is the right molecule, which purity alone does not. - **Research use only. **A statement about how a product is marketed, not a quality certification. See [are peptides legal](https://peptidelibrary.app/blog/are-peptides-legal). ## Evidence terms - **Preclinical. **Before human trials — cell culture and animal work. Most research peptides never leave this stage. - **Phase 1 / 2 / 3. **Human trial stages: safety and dosing, then efficacy signal, then confirmation at scale. - **Endpoint. **What a trial set out to measure. A missed primary endpoint means the study did not show what it was designed to show. - **Dose conversion. **Translating an animal dose to a human one, done by body surface area rather than weight. Skipping that step inflates the result several-fold — see [BPC-157](https://peptidelibrary.app/blog/bpc-157-dosage). > **Research and educational use only. **Peptide Library is an independent research and comparison platform and does not sell peptides. Nothing here is medical advice, dosing guidance, or a recommendation to administer any substance to a person or an animal. Consult a licensed clinician for anything concerning human health. ## Sources - [Hoofnagle AN, et al. Recommendations for the generation, quantification, storage and handling of peptides used for mass spectrometry-based assays](https://pmc.ncbi.nlm.nih.gov/articles/PMC4830481/) Note: Dose figures are left out of this Markdown version. See the cited sources.