Any of a number of acids containing boron and oxygen. Used without qualification the term applies to the compound H₃BO₃ (which is also called orthoboric acid or, technically, trioxoboric(III) acid). This is a white or colourless solid that is soluble in water and ethanol; triclinic; r.d. 1.435; m.p. 169°C. It occurs naturally in the condensate from volcanic steam vents (suffioni). Commercially, it is made by treating borate minerals (e.g. kernite, Na₂B₄O₇.4H₂O) with sulphuric acid followed by recrystallization.

In the solid there is considerable hydrogen bonding between H₃BO₃ molecules resulting in a layer structure, which accounts for the easy cleavage of the crystals. H₃BO₃ molecules also exist in dilute solutions but in more concentrated solutions polymeric acids and ions are formed (e.g. H₄B₂O₇; pyroboric acid or tetrahydroxomonoxodiboric(III) acid). The compound is a very weak acid but also acts as a Lewis acid in accepting hydroxide ions:

If solid boric acid is heated it loses water and transforms to another acid at 300°C. This is given the formula HBO₂ but is in fact a polymer (HBO₂)_n. It is called metaboric acid or, technically, polydioxoboric(III) acid.

Boric acid is used in the manufacture of glass (borosilicate glass), glazes and enamels, leather, paper, adhesives, and explosives. It is widely used (particularly in the USA) in detergents, and because of the ability of fused boric acid to dissolve other metal oxides it is used as a flux in brazing and welding. Because of its mild antiseptic properties it is used in the pharmaceutical industry and as a food preservative.