Hard water
also: hardness
Water high in dissolved calcium and magnesium; good minerals aid extraction, but excess scales machines.
Hard water is water with a high concentration of dissolved minerals, chiefly calcium and magnesium. It sits at the center of a tension every coffee drinker faces: those same minerals both help and hurt.
On the helpful side, calcium and magnesium actively grab onto flavor compounds and pull them out of the grounds, so some hardness improves extraction. Pure distilled water, by contrast, tastes flat and brews weakly. This mineral content is measured as general hardness (GH).
On the harmful side, hard water leaves behind scale: hard mineral deposits, mostly limescale, that build up inside kettles, espresso boilers, and pipes when water is heated. Scale clogs machines, ruins heat transfer, and eventually kills equipment, which is why descaling exists and why espresso owners watch their water closely.
Hardness is separate from alkalinity, the buffering side of water chemistry, though they often rise together. The practical sweet spot balances enough minerals for flavor against enough restraint to protect gear. See water-101 for the fundamentals and water-recipes for tuning your own.