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]

WordNet (r) 3.0 (2006):

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