Glossary

Plain-English definitions of the optic and firearm terms we use across the site — so you can shop with confidence even if you're new to this.

Optic types

Red dot sight: A non-magnified (1×) sight that projects an illuminated aiming dot you simply place on the target. Fast and forgiving up close, and you keep both eyes open.
Reflex sight: The most common kind of red dot — a single lens reflects the dot back to your eye. "Open" designs have an exposed emitter (lighter, common on pistols); "enclosed" designs seal it in a tube (more durable and weather-resistant).
Holographic sight: A red-dot-class sight (1×) that projects a laser-driven hologram reticle (e.g., EOTech). Stays usable even if the front window is partly blocked; favored for fast, close work.
Prism scope: A compact scope with fixed low magnification (1–5×) and an etched glass reticle that works even if the battery dies. Often easier for shooters with astigmatism than a red dot.
LPVO (Low-Power Variable Optic): A variable riflescope that starts near 1× (e.g., 1–6×, 1–8×) — it behaves like a red dot up close and zooms in for distance. The do-it-all carbine optic.
Riflescope: A magnified telescopic sight (fixed or variable power) for medium-to-long range — the right tool when the target is far enough to need glass.
Magnifier: A fixed-power magnifier (usually 3× or 5×) that flips in behind a red dot to add reach, then swings aside for close range.
Thermal optic: Detects heat instead of light, so it "sees" in total darkness, smoke, or light cover. Used for night hunting and security.
Night vision: Lets you aim in the dark by amplifying and processing light, and it works two ways. Passive (non-illuminated) units amplify existing ambient light — moonlight or starlight — and emit nothing, so no one can detect you using them; the classic image-intensifier "tube" units (like a PVS-14) are passive. Active (illuminated) units add an infrared (IR) illuminator that floods the scene with light invisible to the naked eye, so they work even in total darkness — though other night-vision users can spot the IR beam. Most digital NV optics rely on an IR illuminator.
Clip-on: A thermal or night-vision device that mounts in front of your existing day optic — adding night capability without removing or re-zeroing your scope.

Mounting & fitment

Footprint: The standardized screw-and-recoil-lug pattern a pistol red dot uses to bolt down. Your optic and your gun's mounting surface must share a footprint — directly, or bridged by an adapter plate. It's the single biggest thing that decides what fits.
Common footprints: The main pistol-red-dot patterns: Trijicon RMR, RMSc (the compact "Shield" cut), Leupold DeltaPoint Pro (DPP), Aimpoint ACRO, and the Holosun-K. Optics and slide cuts are described by which of these they use.
Adapter plate: A small plate that bridges your pistol's slide (or its optic system) to a specific red-dot footprint — how a non-matching dot still mounts. On each firearm page we name the exact plate an optic needs.
Optic cut (slide cut): A machined recess in a pistol slide that accepts a red dot — directly or via a plate. A slide can be factory-cut or milled aftermarket.
Optics-ready (MOS / OSP / OR): A firearm sold ready to mount an optic, usually with a cut and often included plates. MOS is Glock's system, OSP is Springfield's; other makers use their own labels (OR, TORO, C.O.R.E., RXM).
Picatinny rail (MIL-STD-1913): The standardized slotted rail on most rifles and many shotguns that optics and mounts clamp onto. It's a near-universal mount — which is why a rifle "fits" so many different optics.
Scope rings / mount: Clamp a tube-style scope onto a Picatinny rail. They have to match the scope's tube diameter, so picking the right rings is part of mounting any scope.
Tube diameter: The main-tube size of a scope — usually 1 inch, 30mm, or 34mm. Your rings must match it; larger tubes can allow more adjustment range.
Dovetail: An angled rail found on many rimfire and air rifles (e.g., a 3/8″ dovetail). Often adapted to Picatinny so standard optics can mount.
Co-witness: When your iron sights and red dot line up in the same view — "absolute" (dot sits on the irons) or "lower 1/3" (dot above them) — so you can still aim if the optic ever fails.

Scope & reticle specs

MOA (Minute of Angle): An angular unit of measure — about 1 inch at 100 yards (≈2 inches at 200, and so on). Used for red-dot size ("2 MOA dot"), reticle marks, and scope adjustment clicks.
MIL (Milliradian): Another angular unit: 1 MIL ≈ 3.6 inches at 100 yards (and equals 3.43 MOA). Common on precision scopes. MIL and MOA do the same job in different units — just don't mix them between your reticle and turrets.
FFP (First Focal Plane): The reticle grows and shrinks with magnification, so its holdover and ranging marks stay accurate at any power. Preferred for precision and variable-distance shooting.
SFP (Second Focal Plane): The reticle stays one apparent size at all magnifications; its hold marks are only exact at one power (usually max). Simpler, and common on LPVOs and hunting scopes.
Magnification: How much closer the optic makes the target appear — 1× (none, like a red dot), or a range such as 1–6× or 5–25×.
Objective lens: The front lens; its diameter in millimeters (the "×56" in "5–25×56") affects brightness and field of view — bigger gathers more light but adds size and weight.
Dot size (MOA): How big a red dot appears: smaller (1–2 MOA) is precise for distance, larger (6+ MOA) is faster to find up close. A trade-off, not a quality rating.
Reticle: The aiming pattern you see — a simple dot, a crosshair, or a marked grid (BDC / MIL / MOA) for holdovers and ranging.
BDC (Bullet Drop Compensator): A reticle with stacked holdover marks calibrated to a cartridge's drop, so you can aim for distance without dialing the turret.
Holdover: Aiming above the target (or into the wind) to compensate for bullet drop or drift, instead of dialing the scope's turrets. Reticle marks — BDC, MIL, or MOA — give you the reference points for known distances.
Parallax: Apparent reticle "float" when your eye isn't centered behind the scope. Fixed on most low-power optics; adjustable (side-focus) on higher-power scopes to remove it.
Eye relief: How far your eye sits behind the optic while still seeing a full image. Generous eye relief matters on hard-recoiling guns and scout-style forward mounts.
Illuminated reticle: A battery-lit reticle for low light or against dark targets; switch it off to save battery in good light.

Suitability terms (badges you'll see on the site)

Effective range: The practical distance a gun and cartridge stay useful — we derive it from caliber and barrel length. It's what tells you whether a red dot, an LPVO, or a big scope is the right call.
Direct Fit: On a firearm page, this badge means the optic mounts to your gun with no adapter needed.
Needs Mount: The optic fits, but through an adapter plate or scope rings — and we name the exact part you'll need.
Ideal Magnification: The optic's power suits your firearm's effective range — neither over-scoped (more zoom than the distance calls for) nor under-scoped (not enough reach).
Close Range: A 1× optic (like a red dot) flagged as a deliberate close-range choice on a gun whose range could use more reach. Useful — just not a distance tool.

Firearm basics

Platform / pattern: A firearm's family or lineage — AR, AK, 1911, 2011 — which cuts across type (rifle vs. pistol) and largely determines how optics mount.
Caliber / chambering: The cartridge a firearm fires (e.g., 9mm, 5.56 NATO, .308 Winchester). It changes suitability but not fitment — an optic mounts the same regardless of chambering.
Rimfire: A cartridge whose primer is built into the rim of the case (e.g., .22 LR, .22 WMR, .17 HMR) rather than a separate centerfire primer. Cheap and low-recoil but not reloadable — and rimfire rifles often use a dovetail instead of a Picatinny rail.
Barrel length: Affects velocity and therefore effective range — the same cartridge reaches farther from a longer barrel, which can change the ideal optic.
Carbine: A rifle with a shorter barrel and overall length than a full-size rifle — handier in tight spaces and lighter to carry, for a small trade-off in velocity. Most modern defensive rifles (e.g., a 16″ AR-15) are carbines.
PCC (Pistol-Caliber Carbine): A carbine chambered in a handgun cartridge — usually 9mm. Low recoil, cheap to shoot, and easy to suppress; popular for training, competition, and home defense, and it often shares magazines with a matching pistol.
Large-Format Pistol: A pistol built on a rifle-style platform but with a short barrel and no shoulder stock — AR and AK "pistols," or oversized handguns like the CZ Scorpion. They wear a Picatinny rail (so they mount rifle optics), but their short barrels mean a shorter effective range.
Bullpup: A layout with the action set behind the trigger, giving a shorter overall rifle for the same barrel length (e.g., Tavor, PS90).