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, tart character of coffee, sometimes reminiscent of citrus, berries or wine. It can be pleasant or excessive depending on the coffee and preparation. Perceived acidity is not the same as a measured pH or a prediction of digestive effects.
Arabica: One of the main cultivated coffee species, valued for its diverse aromas, sweetness and acidity. Arabica generally favours cooler growing conditions and can be more vulnerable to pests and disease than Robusta. Both species can produce carefully grown, high-quality coffee.
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 relationship between dry coffee and either brewing water or finished espresso, measured by weight. For filter coffee, 1:16 means 1 gram of coffee to 16 grams of water added. For espresso, 1:2 usually means 1 gram of dry coffee to 2 grams of beverage in the cup. Always check which convention a recipe uses. See also: Brew ratio explained.
Burr grinder: A grinder that crushes coffee between two abrasive surfaces (burrs) to produce particles of consistent size. The adjustable gap gives more control over grind size than a blade grinder. 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 dissolves into the brewed drink. A refractometer and beverage weight can help estimate it. Around 18–22% is a traditional reference range, not a universal quality boundary: even extraction and the taste in the cup matter more than a single target.
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 seeds, prepared through processing and drying. Packaging, moisture, temperature and storage time affect how well they retain quality. A long storage life is not guaranteed simply because the coffee is unroasted.
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: Processing in which the cherry skin is removed and some fruit mucilage remains during drying. Colour names such as yellow, red and black describe producer-specific approaches and are not universal grades or fixed quantities of mucilage. 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: Processing coffee by drying the whole cherry before removing the dried fruit layers. Fermentation and drying conditions influence the flavour; natural coffees can have fruit-forward aromas and substantial body, but the result varies with the coffee and process. 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 bed of ground coffee compressed in an espresso basket, or the spent grounds after brewing. Prepare it evenly before extraction. Cracks, holes or wetness after brewing alone do not reliably diagnose the extraction; use taste and observed flow too. 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 recognised for distinctive qualities that create added value. The SCA’s current approach considers more than a single cupping score; the familiar 80-point threshold is a historical shorthand, not a complete definition. Taste, provenance and the coffee experience all matter. See also: What is specialty coffee?.
Tamping: Compressing grounds evenly in an espresso basket before brewing. Keep the tamper level and press until the bed is compact. Consistency and distribution matter more than chasing a precise force in kilograms. 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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