chiefly British English spelling of saltpeter (q.v.); for ending, see -re.
"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.