The Collaborative International Dictionary of English v.0.48:

Saltpeter \Salt`pe"ter\, Saltpetre \Salt`pe"tre\,
   (s[add]lt`p[=e]"t[~e]r), n. [F. salp[^e]tre, NL. sal petrae,
   literally, rock salt, or stone salt; so called because it
   exudes from rocks or walls. See Salt, and Petrify.]
   (Chem.)
   Potassium nitrate; niter; a white crystalline substance,
   KNO3, having a cooling saline taste, obtained by leaching
   from certain soils in which it is produced by the process of
   nitrification (see Nitrification, 2). It is a strong
   oxidizer, is the chief constituent of gunpowder, and is also
   used as an antiseptic in curing meat, and in medicine as a
   diuretic, diaphoretic, and refrigerant.
   [1913 Webster]

   Chili salpeter (Chem.), sodium nitrate (distinguished from
      potassium nitrate, or true salpeter), a white crystalline
      substance, NaNO3, having a cooling, saline, slightly
      bitter taste. It is obtained by leaching the soil of the
      rainless districts of Chili and Peru. It is deliquescent
      and cannot be used in gunpowder, but is employed in the
      production of nitric acid. Called also cubic niter.

   Saltpeter acid (Chem.), nitric acid; -- sometimes so called
      because made from saltpeter.
      [1913 Webster]