Lewis & Short

† nĭtrum, i, n., = νίτρον, also called sal nitrum,

  1. I. native mineral alkali, native soda, natron; found chiefly in Media, Egypt, Thrace, and Macedonia, Plin. 31, 10, 46, § 106: flos nitri, Vitr. 7, 11, 1; and used for washing with: laveris te nitro, Vulg. Jer. 2, 22; Isid. Orig. 16, 2.
    Hence,
  2. II. Transf.: censuram lomentum aut nitrum esse, Cael. ap. Cic. Fam. 8, 14, 4.