nitre (saltpetre) Mineral, KNO₃; sp. gr. 1.9–2.3; hardness 2; orthorhombic; white, grey, reddish-brown, or lemon-yellow; vitreous lustre; crystals acicular, usually forms granular crusts and uneven masses; occurs in very arid conditions where vegetation is very sparse, in association with evaporite minerals of the arid desert type (e.g. soda nitre, gypsum, halite, and occasionally iodates), and little precipitation is required to wash an encrustation of nitre into hollows where it forms saltierras or massive deposits. It is used as a fertilizer.

nitre †sodium carbonate, (now) saltpetre XIV; †supposed nitrous element in air or plants XVII. — (O)F. — L. nitrum — Gr. nitron, of Sem. or Egyptian orig. (cf. Heb. netr natron, Egyptian ntr); cf. NATRON. The comb. form is nitro-.
So nitric (-IC) applied to an acid produced by the treatment of nitrates with sulphuric acid. XVIII. — F. nitrique. nitrate (-ATE2) XVIII. — F. nitrous (-OUS) XVII. — L. nitrōsus; later — F. nitreux.

nitre an early name for natron or saltpetre; used allusively with reference to natron employed as a cleansing agent, as in Jeremiah 22:2.

From the 17th century, nitre was also the name of a supposed volatile substance related to or present in saltpetre, formerly presumed to be present in the air and rain. It was used with reference to the use of saltpetre in gunpowder, or to the supposition that thunder and lightning were caused by nitre in the air.