The Collaborative International Dictionary of English v.0.48:

Cannula \Can"nu*la\, n. [L. cannula a small tube of dim. of
   canna a reed, tube.] (Surg.)
   A small tube of metal, wood, or India rubber, used for
   various purposes, esp. for injecting or withdrawing fluids.
   It is usually associated with a trocar. [Written also
   canula.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

cannula
    n 1: a small flexible tube inserted into a body cavity for
         draining off fluid or introducing medication