A
- Acid Wash
- A deep-clean process using diluted acid to strip a thin layer of plaster or remove stubborn staining and light scale, exposing fresh surface underneath — different from the muriatic acid used for routine pH/alkalinity adjustment.
- Air Relief Valve
- A small valve on a filter tank that releases trapped air pressure before you open the tank for cleaning or maintenance.
- Algaecide
- A supplementary chemical that helps prevent or treat algae, used alongside chlorine rather than as a substitute for it.
- Alkalinity (Total Alkalinity)
- A measure of water's ability to resist pH change — it acts as a buffer, keeping pH stable rather than swinging with every addition. See our chemistry cheat sheet.
- Automatic Pool Cover
- A motorized cover that opens and closes on tracks at the push of a button, doubling as both a safety barrier and a heat/evaporation cover on many models. See our pool covers guide.
B
- Backwashing
- Reversing water flow through a sand or DE filter to flush trapped debris out through a waste line. See our pool filters guide.
- Bather Load
- The number of swimmers using a pool over a given period — a major factor in how much sanitizer and how often testing is actually needed, since more swimmers means more organic contaminants introduced.
- Biofilm
- A slimy layer of bacteria and organic material that can form on pool surfaces and inside plumbing, often the underlying cause of a pool that keeps clouding up or smelling off despite normal chemical readings.
- Booster Pump
- A secondary, smaller pump dedicated to powering a pressure-side cleaner or certain water features, separate from the main filtration pump.
- Borates (Borax)
- A pH buffer and mild algae inhibitor added to some pools, valued for helping stabilize pH and giving water a noticeably softer feel.
- Breakpoint Chlorination
- The chlorine level high enough to fully oxidize chloramines and organic contaminants in one dose, rather than just keeping up with routine demand. See our shocking guide.
C
- Cal-Hypo (Calcium Hypochlorite)
- A common granular shock chemical — strong and generally the cheapest shock option, but raises calcium hardness over time and must be pre-dissolved before adding. See our shocking guide.
- Calcium Chloride
- The chemical used to raise calcium hardness in pool water. See our chemistry cheat sheet.
- Calcium Hardness
- The concentration of dissolved calcium in pool water — too low causes corrosive water, too high causes scaling. See our chemistry cheat sheet.
- Check Valve
- A one-way valve that lets water flow in only one direction through the plumbing, commonly used to prevent backflow into a booster pump line or heater.
- Chlorine Demand
- The amount of chlorine consumed reacting with contaminants in the water before any is left over as free chlorine — high chlorine demand means a dose disappears fast without raising the reading much.
- Chlorine Lock
- A situation where cyanuric acid (stabilizer) has built up so high that chlorine's effectiveness drops sharply even though the free chlorine reading looks normal.
- Chloramines (Combined Chlorine)
- Chlorine that has already reacted with contaminants and is no longer available to sanitize — the actual cause of a strong chlorine smell. See our chlorine smell guide.
- Clarifier
- A chemical that coagulates very fine suspended particles into larger clumps a filter can catch more easily. See our cloudy pool after shock guide.
- Coping
- The capping material around the edge of a pool shell, both finishing the edge visually and protecting the shell's top edge from the elements.
- Cyanuric Acid (CYA / Stabilizer)
- A chemical that protects chlorine from breaking down in sunlight, but reduces chlorine's effectiveness at high levels — see "chlorine lock" above.
D
- DE (Diatomaceous Earth)
- A fine, naturally occurring mineral powder used as filter media, capable of trapping extremely small particles. See our pool filters guide.
- Deck Drain
- Drainage built into a pool deck to carry away rain and splashed water, preventing pooling that can undermine the deck or wash debris back into the pool.
- Dichlor (Sodium Dichlor)
- A stabilized granular chlorine shock that dissolves easily without pre-mixing, though repeated use steadily raises cyanuric acid. See our shocking guide.
- Dry Acid (Sodium Bisulfate)
- A granular acid used to lower pH and alkalinity — easier to store and handle safely than liquid muriatic acid. See our chemistry cheat sheet.
E
- Enzyme Cleaner
- A non-chlorine product that breaks down oils, lotions, and other organic residues that chlorine alone doesn't fully handle, often used to reduce scum lines and foam-causing surfactants. See our pool foam guide.
- Equalizer Line
- A secondary skimmer line lower down that prevents the skimmer from running dry if water level drops too far — a common leak point in its own right. See our skimmer leak guide.
F
- Fiberglass Pool
- A pool built from a pre-manufactured fiberglass shell, installed as a single piece — typically faster to install than gunite and less prone to algae taking hold on its smooth surface.
- Filter Media
- The material inside a filter that actually traps debris — sand, cartridge fabric, or DE powder, depending on filter type. See our pool filters guide.
- Flocculant (Floc)
- A chemical that clumps fine particles together so heavily they sink to the floor for vacuuming, rather than being filtered — see our cloudy water guide.
- Free Chlorine
- Chlorine that is still active and available to sanitize the water, as opposed to chlorine already used up (combined chlorine). See our chemistry cheat sheet.
- Freeze Protection
- Devices or procedures (freeze guards, auto-run controllers, equipment draining) that prevent pipes and equipment from freezing and cracking in cold weather. See our climate zone closing guide.
G
- Green Algae
- The most common pool algae — free-floating and giving water a green, hazy look. Clears with proper brushing, shock, and filtration. See our green pool guide.
- Gunite / Shotcrete
- A concrete-based pool construction method where a cement mixture is sprayed over rebar to form the shell, then finished with plaster, pebble, or tile — the most common inground pool construction in the US.
H
- Heat Exchanger
- The component inside a heater or heat pump that transfers heat into the pool water — titanium versions resist salt-system corrosion far better than older copper/cupronickel designs. See our pool heaters guide.
- Heat Pump
- A pool heater that extracts ambient heat from the air and transfers it into the water, far cheaper to run than gas but slower and less effective in cold weather. See our pool heaters guide.
I
- Impeller
- The spinning component inside a pump that actually moves water — a worn, cracked, or clogged impeller is a common cause of reduced flow. See our pump and filter troubleshooting guide.
- Infinity Pool (Vanishing Edge)
- A pool with one or more edges designed so water appears to disappear over the horizon, actually flowing into a catch basin and being recirculated back.
L
- Lap Pool
- A long, narrow pool designed primarily for swimming laps rather than general recreation.
- Langelier Saturation Index (LSI)
- A calculation combining pH, calcium hardness, alkalinity and temperature to predict whether water is scale-forming, corrosive, or balanced. See our chemistry cheat sheet.
- Lithium Hypochlorite
- A fast-dissolving chlorine shock that doesn't raise calcium hardness or CYA, gentle on vinyl liners — but typically the most expensive shock option. See our shocking guide.
M
- Main Drain
- The drain at the deepest point of a pool that pulls water into the circulation system — modern installations require anti-entrapment covers by code.
- Metal Sequestrant
- A chemical that binds dissolved metals (iron, copper, manganese) in the water so they don't oxidize and stain surfaces or discolor water.
- Multiport Valve
- The valve on top of a sand or DE filter that switches between filter, backwash, rinse, waste, and other modes. See our pool filters guide.
- Muriatic Acid
- A liquid acid used to lower both pH and alkalinity — faster-acting than dry acid but requires more careful handling. See our chemistry cheat sheet.
O
- ORP (Oxidation-Reduction Potential)
- A measurement of water's sanitizing strength, used mainly by automated controllers as an alternative or supplement to a simple free chlorine reading.
- Oxidation
- The chemical process by which chlorine or shock breaks down organic contaminants and chloramines in the water. See our chemistry cheat sheet.
P
- pH
- A measure of how acidic or basic the water is, on a scale of 0-14 — directly affects chlorine's effectiveness and swimmer comfort. See our chemistry cheat sheet.
- Phosphate Remover
- A chemical that removes phosphates from pool water — phosphates are an algae nutrient source, so some owners use this proactively in algae-prone pools.
- Pink Slime
- A pink or reddish slimy growth (actually a bacteria, not an algae) that tends to appear in fittings, skimmers, and other damp corners — treated with a thorough clean and shock rather than algaecide.
- Plunge Pool
- A small, deep pool intended for cooling off or soaking rather than swimming laps or general recreation.
- Pool Alarm
- A device that detects a disturbance in the water's surface, or a wearable version for children, and sounds an alert if someone or something enters the pool unexpectedly. See our pool accessories guide.
- Pressure Gauge
- The gauge on a filter that reads system pressure — the main early warning sign a filter needs cleaning. See our pressure gauge troubleshooting guide.
- Pressure-Side Cleaner
- An automatic cleaner powered by water pressure from the return line or a dedicated booster pump, sweeping debris into its own attached bag. See our cleaning robots guide.
R
- Reagent Test Kit
- A liquid drop-based test kit using color-matching chemistry — meaningfully more accurate than test strips, especially for distinguishing free from combined chlorine. See our pool accessories guide.
- Return Jet
- The fitting where filtered, treated water re-enters the pool, positioned and angled to help circulate water evenly across the whole pool.
- Robotic Pool Cleaner
- A self-contained automatic cleaner with its own pump and filter, requiring no connection to the pool's main circulation system. See our cleaning robots guide.
S
- Safety Cover
- A cover specifically rated and anchored to support real weight — the only cover type that should be relied on for child or pet safety. See our pool covers guide.
- Salt Chlorinator (Salt Water Generator)
- A device that converts dissolved salt into chlorine through electrolysis, providing continuous sanitizing without manual dosing. See our salt vs chlorine guide.
- Scale
- Calcium deposits that build up on pool surfaces and equipment when water is scale-forming (high pH, high calcium hardness, or both). See our chemistry cheat sheet.
- Shock (Superchlorination)
- A deliberate, temporary spike in chlorine level high enough to reach breakpoint chlorination and clear contaminants. See our shocking guide.
- Skimmer
- The inlet at the water's surface that pulls in floating debris before it can sink, along with the water being drawn into circulation — a common leak point. See our skimmer leak guide.
- Soda Ash
- A chemical used to raise pH directly, with a smaller secondary effect on alkalinity. See our chemistry cheat sheet.
- Sodium Bicarbonate (Baking Soda)
- The standard chemical used to raise total alkalinity. See our chemistry cheat sheet.
- Solar Cover
- A lightweight bubble-style cover that floats on the surface, trapping solar heat and cutting evaporation dramatically. See our pool covers guide.
- Suction-Side Cleaner
- An automatic cleaner that connects to the pool's skimmer or a dedicated suction line, using the main pump's suction to move around and vacuum debris through the filter. See our cleaning robots guide.
- Surfactant
- A soap-like compound (from sunscreen, lotion, some algaecides) that reduces water's surface tension and is the main cause of pool foam. See our pool foam guide.
T
- Total Dissolved Solids (TDS)
- The combined concentration of all dissolved minerals and chemicals in pool water — very high TDS is one reason to partially drain and refill a pool.
- Test Strips
- Simple dip-and-read strips for a quick chemistry check — fast and cheap, but less accurate than a liquid reagent kit, especially for chlorine. See our pool accessories guide.
- Tile Line
- The band of tile at the waterline, prone to scum buildup from oils and lotions — a weekly wipe-down keeps it from becoming a visible ring.
- Trichlor
- A stabilized chlorine tablet or puck form, slow-dissolving and commonly used in floating dispensers or automatic feeders — like dichlor, repeated use gradually raises cyanuric acid. See our shocking guide.
- Turbidity
- A measure of water clarity — the technical term for what's causing water to look cloudy or hazy, whether from fine particles, algae, or precipitated minerals. See our cloudy water guide.
- Turnover Rate
- The time it takes for the filtration system to circulate a volume of water equal to the entire pool, typically targeted at once every 8 hours. See our pool filters guide.
V
- Vacuuming
- Physically removing settled debris from the pool floor, either manually with a pole-mounted vacuum head or automatically via a robotic or suction/pressure-side cleaner. See our pool accessories guide.
- VGB / Anti-Entrapment Drain Cover
- Drain covers and main drain designs compliant with the Virginia Graeme Baker Pool and Spa Safety Act, preventing the suction entrapment hazard older single-drain systems could pose.
- Vinyl Liner
- A flexible vinyl sheet fitted to a pool's frame or structure, common on above-ground pools and some inground pools — more prone to punctures and tears than gunite or fiberglass but often cheaper to install and repair.
W
- Winterizing
- Preparing a pool for the off-season — draining equipment, protecting plumbing from freeze damage, and fitting a winter cover. See our closing a pool guide.