saltpeter

noun

1. the form of potassium nitrate, KNO3, that occurs naturally, used in the manufacture of fireworks, fluxes, gunpowder, etc.; niter.
2. Chile saltpeter.

n.“potassium nitrate,” c.1500, earlier salpetre (early 14c.), from Old French salpetre, from Medieval Latin sal petrae “salt of rock,” from Latin sal “salt” (see salt (n.)) + petra “rock, stone” (see petrous). So called because it looks like salt encrusted on rock.

1. See potassium nitrate.