saltpetre
noun(US English saltpeter)
[uncountable]a white powder used for preserving food and making matches and gunpowder 硝石；钾硝；硝酸钾 Word Originlate Middle English: from Old French salpetre, from medieval Latin salpetra, probably representing sal petrae ‘salt of rock’ (i.e. found as an encrustation). The change in the first element was due to association with salt.