A Homeowner’s Glossary of Plumbing Parts and Terms

A plumber on a service call rattles off “escutcheon,” “dielectric union,” “ferrule,” and “flood rim” in one breath, and most homeowners just nod. The words are not hard once you know them, but no one teaches them in order. This glossary fixes that. Instead of a flat A-to-Z list, the terms below are grouped by where they live in your house: the supply side that brings water in, the drain side that carries it out, the fixtures you touch every day, the small parts that seal and fasten everything together, and the labels you read at the store. Find the area you are dealing with, scan the bold terms, and read only the ones you need.

This is a reference, not a how-to. For how the whole system fits together, see our overview guide (001). For the supply side in depth, see 002, and for the drain-waste-vent side, see 003.

How to Use This Glossary

Look for the part of the house your question belongs to, then scan for the bold word. Each entry is a short, plain definition meant to answer “what is that thing” in a sentence or two, not to teach you how to install or repair it.

A quick orientation helps. Your home has two separate water systems that never mix. The supply side is pressurized and pushes clean water to every fixture. The drain side, also called drainage or DWV (drain-waste-vent), is not pressurized; it uses gravity and air to carry used water out. Many confusing terms make instant sense once you know which side they belong to. A part that holds back pressure is almost always supply side. A part shaped to trap air or water, or one with a wide mouth, is almost always drain side.

Where a term touches plumbing code, requirements are set by model codes such as the International Plumbing Code (IPC) and the Uniform Plumbing Code (UPC), but the exact rules vary by jurisdiction. Treat any code-related note here as general, and verify your local code.

Supply-Side Terms: Valves, Lines, and Fittings

These terms describe the pressurized half of your plumbing, from the street to each fixture.

Service line: The single pipe that brings water from the city main (or a private well) into your house. It is the start of everything on the supply side.

Main shutoff (main water valve): The valve that turns off water to the entire house at once. Knowing where yours is matters more than almost any other piece of plumbing knowledge. The full supply path is covered in our supply-system guide (002).

Fixture shutoff (stop valve, angle stop): The small valve under a sink or behind a toilet that shuts off water to that one fixture without affecting the rest of the house. “Angle stop” refers to the common type that turns the supply 90 degrees up to the fixture.

Supply line (supply tube): The short, often flexible, tube that connects a fixture shutoff to the faucet or toilet itself. Many failures show up here because the tube and its connections are the last few inches before the fixture.

Gate valve: An older valve type that opens and closes with a wheel-style handle by raising and lowering an internal “gate.” It is fully open or fully closed and is poor at throttling flow.

Ball valve: A modern shutoff that uses a rotating ball with a hole through it. A quarter turn of the lever goes from fully open to fully closed, which is why a sideways handle usually means “off.”

Pressure-reducing valve (PRV): A valve installed where the supply enters the house that lowers incoming pressure to a safe range when street pressure is too high. What it does and when you need one is covered in 119.

Dielectric union: A fitting used to join two different metals, most often copper to galvanized steel. It inserts an insulating barrier so the metals do not exchange the small electrical current that causes galvanic corrosion at the joint, which is a documented cause of leaks where copper meets steel.

Nipple: A short length of pipe threaded on both ends, used to connect two fittings. Despite the odd name, it is just a stubby connector.

Coupling, elbow, and tee: Standard fittings named for their shape. A coupling joins two straight pipes, an elbow turns a corner, and a tee splits one line into two. For how supply pipe materials differ, see our pipe-materials guide (100).

Drain-Side Terms: Traps, Vents, Stacks, and Cleanouts

These terms belong to the unpressurized half of the system, where gravity and air do the work.

Trap (P-trap): The U-shaped bend under a sink, tub, or shower that holds a small pool of water. That water plug blocks sewer gas from rising into the room. What a P-trap is and how its seal works is covered in full in 004.

Trap seal: The actual standing water inside a trap. It is the seal, not the pipe, that keeps gas out. A trap in a rarely used fixture can lose its seal to evaporation and let odors in.

Trap arm: The section of drainpipe between the trap and the point where it connects to the vented drain line.

Vent (vent stack): The pipe that lets air into the drain system so water can flow and traps are not siphoned dry. The vent stack is the part that usually exits through the roof. Why vents matter is covered in 005.

Stack: A main vertical pipe in the drain system. A “soil stack” carries waste down, and the upper portion often continues as a vent through the roof.

Branch: A smaller drain line that connects an individual fixture or group of fixtures to a larger stack or building drain.

Building drain and building sewer: The building drain is the lowest horizontal drain inside the house that collects from all the branches. The building sewer is the pipe that carries it from the house to the public sewer or a septic system.

Cleanout: A capped access point on a drain line that lets a plumber insert a snake or camera to clear or inspect the pipe. Codes such as the IPC and UPC require cleanouts at certain runs and changes of direction, but the spacing and placement rules vary by jurisdiction, so verify locally.

Air admittance valve (AAV): A one-way valve that lets air into a drain when a traditional roof vent is not practical. Where AAVs are allowed is jurisdiction-dependent and is covered in 190.

Fixture and Faucet Terms

These are the parts of the things you actually touch and use.

Fixture: Any device that delivers or drains water, including sinks, toilets, tubs, showers, and the like. Plumbers use “fixture” as the catch-all word for these end points.

Cartridge: The replaceable internal core of many modern faucets that controls water flow and temperature with a single mechanism. Older faucets instead use a compression design with rubber washers, or a ball design.

Aerator: The small screened tip that threads onto the end of a faucet spout. It mixes air into the stream to reduce splashing and limit flow without weakening how the stream feels.

Escutcheon: The decorative plate or ring that covers the hole where a pipe or valve passes through a wall or the base of a faucet. It is cosmetic, hiding the rough opening behind it.

Flapper: The rubber seal at the bottom of a toilet tank that lifts to release water into the bowl during a flush, then drops to reseal. A worn flapper is a leading cause of a toilet that runs.

Fill valve and flush valve: Inside a toilet tank, the fill valve refills the tank after a flush, and the flush valve is the opening (sealed by the flapper) that lets tank water rush into the bowl.

Trapway: The internal channel inside a toilet bowl that the waste travels through on its way out. A narrow trapway clogs more easily.

Hose bib (sillcock, spigot): The outdoor faucet on the side of your house that a garden hose attaches to. A “frost-free” version is built to resist freezing.

Air gap: A physical vertical space between a water outlet and the flood rim of a basin, sized so contaminated water can never be siphoned back up into clean supply. The EPA describes the air gap as the simplest and most reliable form of backflow protection, generally at least twice the diameter of the outlet and never less than one inch.

Flood rim (flood-level rim): The top edge of a sink or basin where water would overflow. It is the reference line for measuring an air gap.

Fasteners, Seals, and Connection Terms

These small parts do the unglamorous work of holding joints together and keeping them watertight.

Washer: A flat or beveled ring, often rubber, that compresses to seal a joint. The compression washer inside an old faucet is the classic example.

O-ring: A round rubber loop that seats in a groove to seal a moving or removable part, such as a faucet spout that swivels.

Gasket: A flat sealing ring, larger and often firmer than a washer, used between two flat surfaces, such as the seal between a toilet tank and bowl.

Ferrule: A small ring, usually brass or plastic, that compresses around a pipe inside a compression fitting to form a seal without solder or glue. It is common on supply tubes and shutoff valves.

Compression fitting: A joint that seals by tightening a nut that squeezes a ferrule against the pipe. It lets you connect tubing without soldering.

Union: A fitting that joins two pipes and can be unscrewed later without cutting, which makes a section removable for service.

Thread sealant and pipe dope: Materials applied to threaded joints to fill tiny gaps and prevent leaks. PTFE tape (often called by a common brand name) is the thin white tape wrapped around threads for the same purpose.

Plumber’s putty: A soft, moldable sealant used under the base of fixtures and drains to make a watertight seat. It stays pliable rather than hardening.

Slip joint and slip nut: A slip joint connects drainpipe sections under a sink and is held by a slip nut that you can hand-tighten and loosen, which is why under-sink drain assemblies come apart easily.

Measurement and Material Terms You’ll See on Labels

These are the words and numbers printed on fixture boxes, valves, and product pages.

GPM (gallons per minute): The flow rate of a faucet or showerhead, meaning how much water it delivers over time. By federal standard set under the Energy Policy Act of 1992, showerheads are limited to 2.5 gpm. The EPA’s WaterSense program labels showerheads at 2.0 gpm or less and bathroom faucets at 1.5 gpm or less, compared with a standard faucet flow of 2.2 gpm.

GPF (gallons per flush): The water a toilet uses per flush. The federal standard is 1.6 gpf, and WaterSense labeled toilets use 1.28 gpf or less.

PSI (pounds per square inch): The unit for water pressure, meaning the force pushing water through the pipes. Pressure and flow are different measurements, and confusing them is common. The distinction is explained in 006.

WaterSense: An EPA labeling program. The WaterSense mark on a fixture means it met the program’s water-efficiency and performance criteria through independent certification.

Nominal size: The labeled size of a pipe or fitting (such as “half inch”), which is a naming convention rather than a precise measured dimension. Two pipes of the same nominal size can have slightly different actual dimensions depending on the material and standard.

IPS and NPT: Thread and sizing standards you may see on fittings. NPT (National Pipe Thread) is the tapered thread standard common on threaded plumbing connections in the United States.

PEX, copper, PVC, CPVC, ABS: The common pipe materials. PEX is flexible plastic tubing, copper is rigid metal, and PVC, CPVC, and ABS are rigid plastics used in different applications. How they compare and where each is used is covered in our pipe-materials guide (100).

This glossary stays at the level of “what is that,” on purpose. When a term points to a part you might actually service, the safe-to-DIY work lives in the relevant fix guide, and anything involving gas, a water heater’s internals, the main line, or code-required work belongs to a licensed plumber. Knowing the word is the first step; knowing when to stop and call is the next one.

This article is general information, not professional advice. Plumbing requirements and safe practices vary, and code rules differ by location; verify with your local authority or a licensed plumber before acting.

Sources

• EPA WaterSense, Bathroom Faucets: https://www.epa.gov/watersense/bathroom-faucets
• EPA WaterSense, Showerheads: https://www.epa.gov/watersense/showerheads
• EPA WaterSense, Residential Toilets: https://www.epa.gov/watersense/residential-toilets
• EPA, Cross-Connection Control Manual (air gap and backflow): https://www.epa.gov/sites/default/files/2015-09/documents/epa816r03002_0.pdf
• International Code Council, 2018 International Plumbing Code, Section 708 (Cleanouts): https://codes.iccsafe.org/s/IPC2018/chapter-7-sanitary-drainage/IPC2018-Ch07-Sec708.1