The Collaborative International Dictionary of English v.0.48:

Niter \Ni"ter\, Nitre \Ni"tre\, n. [F. nitre, L. nitrum native
   soda, natron, Gr. ?; cf. Ar. nit[=u]n, natr[=u]n natron. Cf.
   Natron.]
   1. (Chem.) A white crystalline semitransparent salt;
      potassium nitrate; saltpeter. See Saltpeter.
      [1913 Webster]

   2. (Chem.) Native sodium carbonate; natron. [Obs.]
      [1913 Webster]

            For though thou wash thee with niter, and take thee
            much soap, yet thine iniquity is marked before me.
                                                  --Jer. ii. 22.
      [1913 Webster]

   Cubic niter, a deliquescent salt, sodium nitrate, found as
      a native incrustation, like niter, in Peru and Chile,
      whence it is known also as Chile saltpeter.

   Niter bush (Bot.), a genus (Nitraria) of thorny shrubs
      bearing edible berries, and growing in the saline plains
      of Asia and Northern Africa.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Nitre \Ni"tre\, n. (Chem.)
   See Niter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nitre
    n 1: (KNO3) used especially as a fertilizer and explosive [syn:
         potassium nitrate, saltpeter, saltpetre, niter,
         nitre]

Easton's 1897 Bible Dictionary:

Nitre
   (Prov. 25:20; R.V. marg., "soda"), properly "natron," a
   substance so called because, rising from the bottom of the Lake
   Natron in Egypt, it becomes dry and hard in the sun, and is the
   soda which effervesces when vinegar is poured on it. It is a
   carbonate of soda, not saltpetre, which the word generally
   denotes (Jer. 2:22; R.V. "lye").