An alternative spelling of “saltpeter”, preferred in England, is saltpetre.
Chemically, saltpeter is potassium nitrate, a compound essential to the making of gunpowder and also of important value as a fertilizer.
The name comes from the Latin sal petrae, “salt of the rock,” so-called because it is sometimes found in nature as an efflorescence on rocks or soil.
However, saltpeter is definitely not to be confused with rock salt, even though it has a saline taste.
Rock salt is merely sodium chloride (table salt) which has been induced to form large, rather than small, crystals.