A hydroxide is a chemical compound that holds an oxygen atom joined to a hydrogen atom — the pair written OH. That OH group is stuck to a metal (or something like a metal), so you get names like sodium hydroxide, calcium hydroxide, magnesium hydroxide. In water, many hydroxides behave as bases: they can cancel acids. You meet the word in school chemistry and on some product labels (drain cleaner, antacid). Sodium hydroxide is the strong, dangerous one people also call lye; it can burn skin. Calcium hydroxide turns up in lime for building and for treating water. Though actually, a hydroxide is not “a kind of water” and not the same as hydrogen. Water is H₂O; a hydroxide has that OH piece as part of a bigger compound. The word is almost only used by science teachers, chemists, and people reading ingredient lists.