saltpeter (US) noun
or British saltpetre /ˈsɑːltˈpiːtə/ or British saltpetre /ˈsɑːltˈpiːtə/ Learner's definition of SALTPETER
[noncount]
: a white powder that exists naturally in some soils and that is used especially as a fertilizer, in medicine, and to make gunpowder硝石；钾硝；硝酸钾