The Collaborative International Dictionary of English v.0.48:

Fistula \Fis"tu*la\ (?; 135), n.; pl. Fistul[ae]. [L.]
   1. A reed; a pipe.
      [1913 Webster]

   2. A pipe for convejing water. [Obs.] --Knight.
      [1913 Webster]

   3. (Med.) A permanent abnormal opening into the soft parts
      with a constant discharge; a deep, narrow, chronic
      abscess; an abnormal opening between an internal cavity
      and another cavity or the surface; as, a salivary fistula;
      an anal fistula; a recto-vaginal fistula.
      [1913 Webster]

   Incomplete fistula (Med.), a fistula open at one end only.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fistula
    n 1: a chronic inflammation of the withers of a horse [syn:
         fistulous withers, fistula]
    2: an abnormal passage leading from a suppurating cavity to the
       body surface [syn: fistula, sinus]