Glossary

Thirty-seven terms, each with a stable link so an article can point straight at one. Regulatory terms are defined by what the document actually requires, which is usually narrower than the way the term is used in advertising. Brand names appear only where a company holds a named approval, never as entries of their own.

510(k) clearance: A premarket submission in which a manufacturer shows FDA that its device is substantially equivalent to a device already legally on the market. Clearance is a comparison, not a demonstration that the device works on its own evidence. Most aesthetic lasers, radiofrequency systems and cryolipolysis devices reach the market this way.
Ablative: A light or energy treatment that removes tissue by heating the water in it until it vaporises, rather than heating tissue without breaking the surface. Ablative treatments leave an open wound and a longer recovery than non-ablative ones. The distinction is about whether tissue is removed, not about how strong the device is.
Apoptosis: Programmed cell death: a cell dismantling itself in an orderly way and being cleared by other cells, rather than rupturing. It is the mechanism described for cryolipolysis, where the fat cell is not destroyed on the spot but is triggered to die and then removed gradually over following weeks.
BDDE: 1,4-butanediol diglycidyl ether, the chemical most commonly used to crosslink hyaluronic acid gels. It ties separate sugar chains into a three-dimensional network, which is what stops a gel from being cleared as quickly as the body’s own hyaluronic acid. Crosslink density and particle size, rather than brand, largely determine how firm a gel is.
Biostimulator: A category name for injectables marketed as prompting the body to make its own collagen rather than adding volume directly; poly-L-lactic acid and calcium hydroxylapatite are the usual examples. The effect appears over weeks to months rather than immediately, and no enzyme reverses it.
Boxed warning: The most serious warning FDA can require on a prescription drug label, set inside a black border at the top of the prescribing information. Every approved botulinum toxin product carries one, covering the possibility that the toxin’s effect spreads beyond the area of injection.
Chromophore: A substance in tissue that absorbs light at particular wavelengths — in skin, mainly melanin, oxyhaemoglobin and water. Every light-based treatment works by choosing a wavelength its intended chromophore absorbs more strongly than everything nearby. Where something else absorbs the same wavelength, that is heated too.
Crosslinking: Chemically tying separate polymer chains into a network. In fillers it turns a soluble sugar chain into a gel that resists the enzymes that would otherwise clear it within days. It slows degradation without preventing it, which is part of why filler fades unevenly rather than on a schedule.
Dermis: The layer of skin beneath the epidermis, holding collagen, elastin, blood vessels, nerves and glands. Most injectable and energy-based aesthetic treatments are aimed at the dermis or the planes below it, because that is where the structural proteins and the vessels are.
Epidermis: The outermost layer of skin, a fraction of a millimetre thick, containing the pigment-producing melanocytes and acting as the body’s barrier. In light-based treatment it is the layer that has to survive intact while energy is delivered to something deeper, which is why cooling exists.
FDA-approved and FDA-cleared: Two routes with two different burdens of proof. Approval — for a drug, or for a high-risk device through premarket approval — requires evidence of safety and effectiveness for a stated use. Clearance requires a showing of substantial equivalence to a product already marketed. Advertising frequently prints the second as though it meant the first.
Fitzpatrick scale: A classification of skin by how it responds to sun exposure rather than by any measurement of pigment. It began in the 1970s as a way of judging how much ultraviolet a person could safely be given in phototherapy; the six-point form the laser literature uses throughout was set out by Fitzpatrick in the Archives of Dermatology in 1988. It is a rough proxy for melanin content, and it was not designed for that job.
Foreign-body reaction: The immune response to material the body cannot break down or remove: cells surround it and wall it off, sometimes producing a firm nodule. It is part of the proposed mechanism of some biostimulator injectables and also part of their recognised complication profile — the same process on both sides of the ledger.
Fractional: Delivering energy in separated columns, leaving untreated skin between them. Healing runs inward from the untreated edges, which is why fractional treatments recover faster than treating a whole surface at once. It also means coverage in a single pass is partial by design rather than by compromise.
Hyaluronic acid: A large sugar chain that occurs naturally throughout the body and binds many times its weight in water. Injectable fillers use a crosslinked form of it. The body’s own enzyme, hyaluronidase, cleaves it — which is why this one filler chemistry can be dissolved and the other filler chemistries cannot.
Hyaluronidase: An enzyme that cleaves hyaluronic acid. FDA-approved hyaluronidase products exist, approved as agents that help other injected fluids disperse and absorb. Using it to dissolve dermal filler, including in an emergency, is an off-label use that FDA has not evaluated for that purpose.
Intense pulsed light (IPL): A broadband flash lamp whose output is shaped by cut-off filters — not a laser. Because it emits a range of wavelengths in a single pulse, several chromophores in the skin absorb from that pulse at once, which makes it inherently the least selective light source in ordinary aesthetic use.
Lot number: The manufacturing batch identifier printed on a vial or syringe and on its packaging. It is the thread connecting a product in a clinic to a recall, a warning letter or a counterfeit investigation, and it is among the first things regulators ask for when a product is suspect.
MedWatch: FDA’s programme for reporting adverse events and product problems with drugs and devices. Members of the public can file directly, not only clinicians. Reporting is voluntary, which is one reason the true rate of a complication in this field is often unknown rather than merely uncertain.
Nanometre: One billionth of a metre, the unit used for the wavelength of light. Device names in aesthetics are frequently wavelengths — 1064, 1550, 1927 — and the number tells you which chromophore the device is aimed at and, roughly, how deep into tissue that light reaches.
Neocollagenesis: The formation of new collagen. It is the outcome claimed for most energy-based tightening treatments and for biostimulator injectables. Demonstrating it in a tissue sample is a different finding from demonstrating a visible change in a person, and the two are often reported as though they were the same.
Neuromuscular junction: The gap where a motor nerve meets a muscle fibre and passes on its signal by releasing acetylcholine. It is where botulinum toxin acts: the toxin interferes with the release of that signal rather than with the muscle itself or with the nerve’s ability to survive.
Off-label use: A licensed clinician using an approved product for a use, a site or a population outside its approved labelling. It is lawful and is often ordinary medical practice. It also means FDA has not assessed that specific use, so the approval provides no evidence for it.
Panniculitis: Inflammation of the fat layer beneath the skin. In cryolipolysis it is a described step in the mechanism rather than a complication: crystallisation triggers cell death, inflammation follows, and the debris is cleared over weeks — which is why the visible result is gradual rather than immediate.
Paradoxical adipose hyperplasia: A complication of cryolipolysis in which the treated fat pad enlarges instead of shrinking, usually becoming apparent months afterwards. It does not resolve on its own and correction is generally surgical. Published incidence figures differ substantially between manufacturer data and independent multicentre series.
Periosteum: The dense membrane covering bone. In facial injection it matters as an anatomical landmark: the literature describes some products as being placed against it, and the plane a product occupies is part of how that product behaves and how long it lasts.
Premarket approval (PMA): The most demanding FDA device pathway, requiring evidence of safety and effectiveness for the stated use rather than a comparison with an existing product. Dermal fillers are class III devices and go through it, which is why “approved” is accurate for fillers and not for most energy-based devices.
Reconstitution: Mixing a freeze-dried drug with a diluent before use. Botulinum toxin products are supplied as a dried powder and made up in the clinic, so how a vial was stored, prepared and handled before injection is part of the condition of the product that reaches a patient.
REMS: A Risk Evaluation and Mitigation Strategy: a programme FDA can require alongside an approval where risks need managing beyond ordinary labelling, typically including a medication guide. FDA’s 2009 action on botulinum toxin products added the boxed warning and required risk management of this kind.
Selective photothermolysis: The principle behind light-based treatment, set out in Science in 1983: choose a wavelength the target absorbs preferentially, and use a pulse shorter than that target’s thermal relaxation time, so heat stays in the target instead of spreading. Selectivity is a calculation about physics, not a safety feature.
SMAS: The superficial musculoaponeurotic system, a fibrous layer lying between the fat under the skin and the deeper muscles of the face. It is the layer surgical facelifts act on, and it is named in the marketing of microfocused ultrasound because that device is described as depositing energy at a depth reaching it.
SNAP-25: A protein at the nerve ending that vesicles of acetylcholine need in order to dock and release their contents. Botulinum toxin type A cleaves it, so the signal cannot be released and the muscle is partly denervated. The prescribing information also notes that reinnervation may occur, slowly reversing the effect.
Subcutaneous: Beneath the skin, in the fat layer between the dermis and the muscle. It is the plane that deoxycholic acid injection and cryolipolysis are aimed at, and the reason both are described as acting on a fat pad rather than as treatments of the skin above it.
Systematic review: A review that states its search method, its inclusion rules and what it excluded, so a reader can see how much of the literature it captured. Read the screened count and the included count before the conclusion: the gap between those two numbers describes the field as clearly as the conclusion does.
Thermal relaxation time: Roughly the time a heated structure takes to shed about half its heat to its surroundings. It scales with the size of the structure, which is why a small target needs a short pulse and a larger one tolerates a longer pulse. It is the second half of selective photothermolysis.
Unit (of botulinum toxin): A measure of biological activity defined by each manufacturer’s own potency assay, using that manufacturer’s vehicle, dilution scheme and laboratory protocol. Because the assays differ, units of one product cannot be compared with or converted into units of another — a statement that appears in the prescribing information itself.
Vascular occlusion: A blood vessel blocked so that the tissue it supplies loses its blood supply. In filler treatment it is the rare complication that matters most, because material entering an artery can cause tissue loss and, where facial vessels connect toward the eye, visual loss. It is a medical emergency.

Terms are defined against the documents named at how we read the evidence. Where a definition rests on a regulatory document rather than on ordinary usage, the entry says which document.

Reviewed: 8 August 2026 · Evidence current to: 8 August 2026