Under-extraction
also: underextraction, underdeveloped
Pulling too little from the grounds; tastes sour, thin, salty, and empty. Fix: finer, hotter, longer.
Under-extraction means you dissolved too little out of the grounds, leaving a lot of good flavor stuck in the spent bed. In yield terms it usually sits below the roughly 18 percent mark.
The acids and bright compounds in coffee dissolve first, while the sweetness and balancing flavors come out a little later. Stop too early and you get the front of that sequence without the rest. The cup tastes sour and sharp (not the pleasant kind), thin in body, sometimes faintly salty, and oddly empty or short on the finish. People often mistake this for “bad beans” when the brew is simply incomplete.
The fixes all push extraction up. Grind finer so water touches more surface area, brew hotter, extend the contact time, or add agitation like a stir or swirl. Often one change is enough. Be careful not to overshoot into over-extraction. If you cannot tell which way to go, sour-vs-bitter is the clearest tasting guide, and why-is-my-coffee-sour walks through it.