The bright, lively quality in a coffee that makes you salivate — analogous to the acidity in a green apple, a citrus fruit, or a glass of crisp white wine. In specialty coffee, acidity is a virtue, not a flaw. It's what separates a cup that tastes alive from one that tastes flat.
Acidity comes from organic acids — citric, malic, phosphoric, acetic, lactic — that survive the roast and dissolve into the brew. Different origins lead with different acids: Kenya is famously phosphoric (blackcurrant, dry tartness), Ethiopia is citric and malic (lemon, peach, jasmine), and naturally processed coffees lean lactic (yogurt, fermented red fruit).
How to recognize good acidity
The mouth response is the giveaway. Real acidity makes you produce saliva — the back of your tongue and the sides of your throat respond. There's brightness, lift, a sense of "the cup is moving forward." It's pleasant, not sharp.
Confused with: astringency (drying, puckering — a defect, not acidity), sourness (lemon-juice front-of-mouth tartness from severe under-extraction), and bitterness (a different sense entirely, from the back palate).
When acidity goes wrong
Under-extracted coffee tastes sour — only the citric and acetic acids dissolved, none of the sweetness or balance behind them. The fix is more extraction (finer grind, longer time, hotter water).
Lower-quality or stale beans can taste flat — the volatile aromatics that gave the acidity its character have faded, leaving just the dull acidic backbone. The fix is fresher, better beans.