The Collaborative International Dictionary of English v.0.48:

mannitol \man"ni*tol\, n. [Mannite + -ol.]
   1. (Chem.) A white crystalline hexose
      (HO.CH2.(CHOH)4.CH2.OH) of a sweet taste obtained from a
      so-called manna, the dried sap of the flowering ash
      (Fraxinus ornus); -- called also mannite, and hydroxy
      hexane. Cf. Dulcite. It is used in pharmacy as
      excipient and diluent for solids and liquids. It is also
      used as a food additive for anti-caking properties, or as
      a sweetener, and, illegally, to "cut" (dilute) illegal
      drugs such as cocaine or heroin.

   Syn: D-mannitol; manna sugar; cordycepic acid; Diosmol;
        Mannicol; Mannidex; Osmiktrol; Osmosal.
        [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

mannitol
    n 1: a diuretic (trade name Osmitrol) used to promote the
         excretion of urine [syn: mannitol, Osmitrol]