Coffee Glossary — Terms Every Home Barista Should Know

Plain-language definitions for the 43 terms you will encounter in specialty coffee — from TDS and extraction yield to channeling, crema and WDT.

Acidity: The bright, lively tartness in coffee, often described as citrusy, fruity or wine-like. A positive quality in specialty coffee that indicates careful growing at altitude and good processing. Not the same as stomach acidity or sourness caused by under-extraction.
Arabica: The higher-quality species of coffee plant, used in almost all specialty coffee. Produces more complex flavours than Robusta but requires higher altitude, more care and is harder to grow at scale. All specialty-grade coffee is Arabica.
Basket: The perforated metal filter that sits inside the portafilter and holds the coffee grounds during espresso extraction. Baskets come in different sizes (15 g, 18 g, 20 g+) and designs — pressurised baskets are more forgiving, non-pressurised baskets demand correct grind and technique. See also: Espresso basket sizes.
Bloom: The initial pour of a small amount of hot water over grounds in filter brewing. Freshly roasted coffee contains CO2 that expands on contact with water, causing the grounds to swell and bubble. Blooming allows this gas to escape before the main brew begins, improving even extraction. See also: What is bloom?.
Body: The physical weight and texture of coffee in the mouth. A full-bodied coffee feels thick and coating — like whole milk. A light-bodied coffee feels closer to tea or water. Body is influenced by brewing method, processing and variety. Immersion brewing (French press) and natural processing both tend to increase body.
Brew ratio: The ratio of dry coffee to water by weight. A 1:15 ratio means 1 gram of coffee per 15 grams of water. Espresso ratios are typically 1:2 to 1:3; filter ratios 1:15 to 1:17. Brew ratio is one of the most direct controls over strength and extraction. See also: Brew ratio explained.
Burr grinder: A grinder that crushes coffee between two abrasive surfaces (burrs) to produce particles of consistent size. Far superior to blade grinders for espresso and filter brewing. Flat burrs and conical burrs are the two main types, each with different grind distribution characteristics. See also: Burr vs blade grinder.
Channeling: When water under pressure finds a path of least resistance through the espresso puck rather than flowing evenly through all the grounds. Channels produce uneven extraction — some areas are over-extracted, others barely touched. Common causes are uneven distribution, tamping defects and damaged pucks. See also: What is channeling?.
Crema: The reddish-brown foam layer on top of a freshly pulled espresso, formed by CO2 dissolved in the coffee under pressure. An indicator of freshness — stale coffee produces little or no crema. Crema volume and persistence are not direct measures of quality, and some excellent espressos have very little. See also: Why espresso has no crema.
Degassing: The release of CO2 from freshly roasted coffee over time. Roasting drives large amounts of CO2 into the bean, which escapes slowly after roasting. Brewing too soon after roasting produces bitter, uneven results. Allowing coffee to rest until degassing slows is called resting. See also: Resting coffee after roast.
Dial in: The iterative process of adjusting grind size, dose, yield, temperature and other variables to achieve a balanced, well-extracted espresso from a specific coffee on specific equipment. Every new bag requires some degree of dialling in because roast level, density and freshness all affect extraction behaviour. See also: How to dial in espresso.
Distribution: How evenly ground coffee is spread across the portafilter basket before tamping. Poor distribution — clumps on one side, gaps on another — leads to channeling and inconsistent extraction. WDT (Weiss Distribution Technique) is the most widely used method to improve distribution. See also: WDT — Weiss Distribution Technique.
Dose: The weight of dry coffee grounds used in a single brew. Espresso doses are typically 18-21 g; filter doses 15-25 g depending on the brewer and recipe. Dose is always measured by weight, not by volume or scoops, for consistency.
Extraction: The process of dissolving soluble compounds from coffee grounds into water. Under-extraction (too little dissolved) produces sour, thin, sharp-tasting coffee. Over-extraction (too much dissolved) produces bitter, harsh, drying coffee. Correct extraction is the goal of dialling in. See also: Brew ratio explained.
Extraction yield: The percentage of the dry coffee dose that was dissolved into the brewed drink. Specialty coffee targets roughly 18-22% extraction yield. Below 18% the coffee is under-extracted; above 22% it is likely over-extracted. Measured precisely with a refractometer and TDS reading.
Fines: The very small coffee particles produced during grinding, even by high-quality burr grinders. Fines extract much faster than coarser particles and can cause bitterness or over-extraction even when the overall grind size is appropriate. Grinders that produce fewer fines tend to produce cleaner, more predictable extractions.
Green coffee: Unroasted coffee beans. Coffee is harvested as a cherry, processed to remove the fruit, and dried before being shipped as green coffee to roasters. Green coffee has a long shelf life (1-2 years) and is stable; roasting is what makes it volatile and perishable.
Grind retention: The amount of coffee left inside a grinder after grinding, either lodged in the burr chamber or sitting in the chute. High retention means stale grounds from a previous session mix with fresh grounds from the current one, affecting flavour and making accurate dosing difficult. See also: Grinder retention explained.
Group head: The part of an espresso machine where the portafilter locks in and through which hot pressurised water flows into the coffee puck. Group head temperature stability is critical — a cold or inconsistent group head produces inconsistent extraction and off flavours.
Honey process: A processing method where the coffee cherry skin is removed but some or all of the sticky fruit mucilage is left on the bean during drying. Yellow honey (less mucilage) is cleaner; red and black honey (more mucilage, slower drying) add sweetness and body. A middle path between washed and natural processing. See also: Honey process explained.
Immersion brewing: A brewing style where coffee grounds are fully submerged in water for the entire brew time, then separated. French press, AeroPress and V60 Switch are immersion methods. Immersion tends to produce more body and less brightness than percolation (pour-over) brewing. See also: French press method.
Microlot: A small, specific batch of coffee from a defined section of a farm or a single producer, picked and processed separately from the main harvest. Produced for quality rather than volume and sold at a premium. Microlots allow precise traceability and often represent the best fruit from a farm in a given season. See also: What is a microlot?.
Mucilage: The sticky layer of fruit flesh surrounding the coffee seed inside the cherry, beneath the outer skin. Removed in washed processing; left on partially in honey processing; left on fully during the early stages of natural processing. Mucilage fermentation contributes sweetness and body to the final cup.
Natural process: A method where the whole coffee cherry is dried without removing any fruit first. The seed ferments inside the cherry over several weeks, absorbing sugars and producing intense fruit flavours — berry, wine, dark chocolate. Produces heavier body and lower acidity than washed processing. See also: Washed vs natural.
Percolation: A brewing method where water passes through coffee grounds and drains, rather than steeping them. V60, Chemex, Kalita Wave and drip machines are percolation brewers. Produces cleaner, brighter cups than immersion brewing by constantly refreshing the water in contact with grounds.
PID: Proportional-Integral-Derivative controller. An electronic temperature regulation system used in espresso machines to hold the boiler or group head at a precise, stable temperature. Machines with a PID produce more consistent shots than those with basic thermostats. See also: PID explained.
Portafilter: The handled filter holder that locks into the group head of an espresso machine and holds the basket containing the coffee grounds. Bottomless (naked) portafilters expose the basket base so you can observe the extraction flow and spot channeling. See also: How to read a bottomless portafilter.
Pre-infusion: A low-pressure wetting of the espresso puck before full extraction pressure is applied. Pre-infusion allows the grounds to saturate gradually and swell, reducing channeling and promoting more even extraction. Some machines do this automatically; others require a flow controller or manual control. See also: What is pre-infusion?.
Pressure profiling: Varying the water pressure during espresso extraction — typically starting low, ramping up, then tapering off at the end — to control how different flavour compounds are extracted over time. Lever machines profile naturally; pump machines require additional hardware to achieve it. See also: Pressure profiling explained.
Puck: The compressed disc of coffee grounds in the portafilter basket after tamping, and the spent disc of grounds remaining after extraction. A well-prepared puck is level, evenly dense and holds its shape during extraction. A channeled or poorly distributed puck will show cracks or holes. See also: Puck prep guide.
Refractometer: An optical instrument that measures how much light bends when passing through brewed coffee, which indicates the concentration of dissolved solids (TDS). Used with dose and yield measurements to calculate extraction yield. A useful tool for dialling in precise recipes.
Resting: Allowing freshly roasted coffee to off-gas CO2 before brewing. Light roasts typically benefit from 7-21 days of rest; darker roasts off-gas faster and are often ready in 3-7 days. Brewing before adequate rest produces flat, uneven extraction and bitter or soapy flavours. See also: Resting coffee after roast.
Ristretto: A short espresso pulled at a lower ratio than standard — typically 1:1 to 1:1.5 (e.g., 18 g in and 18-27 g out). Stops the extraction earlier, producing a more concentrated, sweeter and less bitter shot. Often used as the espresso base in flat whites. See also: Ristretto vs lungo explained.
Roast date: The date the coffee was roasted, printed on specialty bags. The primary freshness indicator for home baristas. Light roasts are typically best 5-21 days after roasting; darker roasts 3-10 days. More meaningful than a best-before date for understanding when a coffee is at its peak. See also: How to read a coffee bag.
Single origin: Coffee sourced from a single farm, cooperative, washing station, region or country, rather than blended from multiple origins. Allows the character of that specific place, variety and processing method to come through clearly in the cup. See also: Single origin vs blend.
Specialty coffee: Coffee that scores 80 points or above on the SCA (Specialty Coffee Association) 100-point scale. The score is assessed by a Q Grader on the unroasted green coffee and reflects origin quality, processing and defect count. The label describes the raw material, not the roasting or brewing. See also: What is specialty coffee?.
Tamping: Compressing the ground coffee in the portafilter basket with a flat disc (tamper) to create a level, dense puck before espresso extraction. Consistent tamping pressure and a level tamp surface reduce channeling. Most baristas aim for around 15-20 kg of pressure applied straight down. See also: Puck prep guide.
TDS: Total Dissolved Solids. A measure of how much material is dissolved in brewed coffee, expressed as a percentage. Specialty filter coffee typically targets 1.2-1.5% TDS; espresso is much higher at 8-12%. Measured with a refractometer. Together with dose and yield, TDS allows calculation of extraction yield.
Terroir: The combination of soil, altitude, climate, rainfall and environment that shapes the flavour character of coffee grown in a specific place. Borrowed from wine vocabulary. The reason an Ethiopian Yirgacheffe and a Brazilian natural from the same variety can taste completely different.
Variety / Cultivar: The specific genetic strain of the coffee plant. Bourbon, Typica, Geisha, SL28, Pacamara and Catuai are varieties of Arabica with different flavour potentials, yield levels and growing requirements. Variety is listed on high-quality specialty bags and is a meaningful indicator of what to expect in the cup. See also: Coffee varieties explained.
Washed process: A processing method where all fruit is removed from the coffee bean before drying, typically by fermenting the beans in water or using mechanical demucilaging to break down the mucilage. Produces clean, transparent cups where the character of the origin, altitude and variety come through clearly. See also: Washed vs natural.
WDT: Weiss Distribution Technique. Using a thin needle (0.3-0.4 mm diameter) to stir and break up clumps in the portafilter basket after dosing and before tamping. Improves distribution of grounds across the basket, reduces channeling and makes extraction more consistent. See also: WDT explained.
Yield: The weight of liquid espresso in the cup after extraction, measured in grams. A 1:2 ratio with 18 g coffee produces a 36 g yield. Always measured by weight rather than volume for consistency, since crema and temperature affect liquid volume but not mass.

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