The Coffee Glossary
Every term you'll meet in our reviews and the wider specialty coffee world — defined in plain English by our test team.
- Bloom
- The initial pour of hot water over fresh grounds that releases trapped CO₂, causing the bed to bubble and swell. A 30–45 second bloom improves evenness of extraction in pour over and French press.
- Burr Grinder
- A grinder that crushes beans between two abrasive surfaces (flat or conical burrs) at a set distance, producing far more uniform particles than a blade grinder. The single most important upgrade for better coffee.
- Channeling
- When water finds a path of least resistance through an espresso puck, over-extracting one spot and under-extracting the rest. Causes sour-bitter shots. Fixed with better distribution and tamping.
- Crema
- The tan foam atop an espresso shot, formed by CO₂ emulsifying with coffee oils under 9 bars of pressure. A marker of fresh beans and proper extraction — not a quality guarantee on its own.
- Dialing In
- The process of adjusting grind size, dose and yield until a coffee tastes balanced. For espresso, typically targeting a 1:2 ratio in 25–30 seconds.
- Dose
- The weight of dry coffee used for a brew. Standard double-shot espresso doses run 16–20g; filter brews typically use 55–70g per liter of water.
- Dual Boiler
- An espresso machine with separate boilers for brewing and steaming, allowing both at once with independent temperatures. The gold standard for milk-drink workflow.
- E61 Grouphead
- The iconic chrome grouphead introduced by Faema in 1961. Thermosyphon circulation keeps it hot; a mechanical lever provides pre-infusion. Found on most prosumer heat-exchanger machines.
- Extraction
- The dissolving of coffee solubles into water. Under-extraction tastes sour and thin; over-extraction tastes bitter and hollow. Ideal extraction yields sweetness and balance, typically 18–22% of the grounds' mass.
- God Shot
- Barista slang for a rare, perfectly balanced espresso shot where sweetness, acidity and body align. What keeps home baristas dialing in.
- Group Head
- The part of an espresso machine where the portafilter locks in and water meets coffee. Its temperature stability largely determines shot consistency.
- Heat Exchanger (HX)
- A boiler design where brew water passes through a tube inside the steam boiler, heating instantly. Enables simultaneous brewing and steaming with one boiler.
- Immersion Brewing
- Any method where grounds steep in the full volume of water (French press, cupping, AeroPress). Produces heavier body than percolation methods like pour over.
- Microfoam
- Steamed milk with bubbles too small to see, giving a glossy, paint-like texture. Required for latte art and the hallmark of proper milk steaming technique.
- Moka Pot
- A stovetop brewer that pushes water through grounds at ~1.5 bar. Produces strong, espresso-adjacent coffee — but not true espresso, which requires ~9 bars.
- PID Controller
- A Proportional-Integral-Derivative controller that holds boiler temperature within ±1°F instead of the ±15°F swing of a basic thermostat. Dramatically improves shot consistency.
- Portafilter
- The handled metal basket-holder that locks into the group head. Commercial standard is 58mm; Breville uses 54mm; some brands use 51mm.
- Pre-infusion
- Wetting the puck at low pressure before full extraction pressure. Reduces channeling and allows finer grinds. Built into E61 groups; programmable on many modern machines.
- Pressurized Basket
- A double-walled filter basket that creates artificial crema through a small exit valve. Forgiving for beginners and pre-ground coffee, but caps flavor quality. Also called a dual-wall basket.
- Puck
- The compressed disc of spent espresso grounds knocked out after a shot. A dry, intact puck is a (loose) sign of good dose and grind.
- Ratio (Brew Ratio)
- The ratio of dry coffee to beverage weight. Espresso: 1:2 (18g in, 36g out) is the modern standard. Filter: 1:15 to 1:17 by weight.
- Retention
- Grounds that stay inside a grinder between doses. High retention wastes coffee and mixes stale grounds into fresh doses. Single-dose grinders minimize it.
- Single Dose Grinding
- Weighing and grinding only the beans needed for one brew, keeping the hopper empty. Maximizes freshness and allows easy switching between coffees.
- Steam Wand
- The articulated pipe on an espresso machine that injects steam into milk. More boiler power means faster texturing and better microfoam.
- Tamping
- Compressing the coffee bed in the portafilter with ~15–30 lbs of even pressure. Level matters far more than force — an angled tamp invites channeling.
- TDS (Total Dissolved Solids)
- The concentration of coffee solubles in a brewed cup, measured with a refractometer. Ideal filter coffee sits around 1.15–1.45% TDS; espresso 8–12%.
- Thermoblock / Thermocoil
- On-demand heating elements that flash-heat water as it flows, enabling 3-second heat-up in machines like the Breville Bambino. Less thermal mass than a boiler but far faster.
- Third Wave Coffee
- The movement treating coffee as a craft product like wine: single origins, light roasts, direct trade and precise brewing. Contrast with first wave (mass-market tins) and second wave (espresso chains).
- V60
- Hario's iconic 60-degree conical dripper with spiral ribs and a large single hole. The reference pour over brewer — flow rate is controlled entirely by grind and pour technique.
- Yield
- The weight of liquid espresso in the cup. Combined with dose and time, one of the three variables baristas adjust when dialing in.