: the monovalent anion OH− consisting of one atom of hydrogen and one of oxygen—called also hydroxide ion
: an ionic compound of hydroxide with an element or group
Any compound with one or more functional groups made up of one atom each of hydrogen and oxygen, bonded together and acting as the hydroxide anion (OH). Hydroxides include the familiar alkalies of laboratory and industrial processes. Those of the alkali metals (lithium, sodium, potassium, rubidium, and cesium), the strongest bases, are the most stable and soluble; those of the alkaline earth metals (calcium, barium, and strontium), also soluble strong bases, are less stable. The hydroxides of most other metals are only slightly soluble but neutralize acids; some are amphoteric, reacting with both acids and bases. In compounds in which OH is un-ionized and covalently bonded (e.g., in methanol, CHOH), it is known as a hydroxyl group.