The pool glossary.
22 plain-English definitions for the chemistry, equipment, and construction terms that come up whenever you own a pool.
All glossary terms
Algaecide
A chemical treatment that kills or inhibits algae growth, used alongside, not instead of, proper chlorine and filtration.
Backwashing
Reversing water flow through a sand or DE filter to flush out trapped debris, the standard cleaning method for those filter types.
Bather load
The number of swimmers using a pool at a given time, one of the biggest short-term drivers of chlorine demand and water cloudiness.
Bonding
The electrical safety practice of connecting all metal pool components, ladders, lights, drain covers, pump motors, together so they share the same electrical potential.
Breakpoint chlorination
The specific chlorine dose that overwhelms all the ammonia and organic contaminants in your water at once, wiping out combined chlorine (chloramines) in a single pass.
Chlorine lock (stabilizer lock)
A popular but misleading term for what happens when cyanuric acid (stabilizer) builds up so high that chlorine becomes too weak to sanitize effectively.
Clarifier
A chemical that coagulates microscopically fine particles into slightly larger clumps small enough to pass through and get caught by the filter.
Combined chlorine (chloramines)
The portion of chlorine in your water that has already reacted with contaminants and lost most of its sanitizing power, better known as chloramines.
Diatomaceous earth (DE)
A fine, fossilized-algae powder used as filter media, prized for trapping the smallest particles of any common home pool filter type.
Flocculant (floc)
A chemical that clumps fine, suspended particles together so they sink to the pool floor for vacuuming, used for a fast, dramatic cloudy-water fix.
Freeboard
The gap between the pool's coping (edge) and the resting water level, which keeps splash-out contained and lets the skimmer weir do its job.
Langelier Saturation Index (LSI)
A calculated water-balance score, not something you test directly, that predicts whether your pool water will lean toward scale-forming or corrosive.
Main drain
The fitting, or pair of fittings, at the pool's deepest point that pulls water from the bottom into circulation alongside the skimmer.
Pebble finish
A durable interior pool surface made of small aggregate pebbles set in cement, prized for its longevity and natural look over plain plaster.
Plaster finish
A traditional smooth, cement-based interior pool surface, the most common and least expensive finish for concrete/gunite pools.
Return jet
The fitting where filtered, treated water re-enters the pool, positioned and aimed to keep water circulating rather than sitting stagnant.
Sequestrant
A chemical that binds dissolved metals in pool water so they stay suspended and invisible instead of oxidizing into visible stains.
Superchlorination
Adding a large, one-time dose of chlorine, well beyond the normal daily range, to rapidly raise free chlorine and knock out contaminants and combined chlorine.
Total dissolved solids (TDS)
The total concentration of everything dissolved in your pool water, minerals, salts, and chemical byproducts combined, measured in parts per million.
Turnover rate
The time it takes a pool's pump and filter to circulate a volume of water equal to the entire pool, the baseline measure of how well a pool filters itself.
Weir
The flap or gate at the mouth of a skimmer that pulls a thin layer of surface water in while keeping debris from floating back out.
Winterizing
Preparing a pool and its equipment for a cold-weather shutdown so freezing water can't crack pipes, pumps, or the pool shell.