Nitre (most English-speaking countries) or Niter (American English) is the mineral form of potassium nitrate, KNO₃, also known as saltpeter in America or saltpetre in other English-speaking countries. Historically, the term "nitre" was not well differentiated from natron, both of which have been very vaguely defined but generally refer to compounds of sodium or potassium joined with carbonate or nitrate ions. Three related minerals are soda nitre (sodium nitrate), ammonia nitre (ammonium nitrate), and strontium nitrate.