noun
: a hygroscopic crystalline compound NaBH 4 that is flammable but remarkably stable in air and decomposes readily in water only above room temperature or in the presence of acid to yield hydrogen and sodium metaborate, that is made in various ways (as from methyl borate and sodium hydride or from diborane and sodium methoxide), and that is used chiefly as a reducing agent for organic compounds — called also sodium tetrahydroborate