noun
limescale; scale; water stain; fur
Refers to mineral deposits left by water, especially on kettles, sinks, or bathroom surfaces. 'Fur' is a British English term for the same buildup.
やかんに水垢がたまっている。
Limescale has built up in the kettle.
お風呂の水垢を落とすのは大変だ。
Removing water stains from the bath is a chore.
湯垢 specifically refers to scale or scum from hot water, often in baths, while 水垢 is more general for cold or hot water deposits.
Compound of 水 (water) and 垢 (dirt, grime), literally 'water grime'.