The Collaborative International Dictionary of English v.0.48:

Sagitta \Sa*git"ta\, n. [L., an arrow.]
   1. (Astron.) A small constellation north of Aquila; the
      Arrow.
      [1913 Webster]

   2. (Arch.) The keystone of an arch. [R.] --Gwilt.
      [1913 Webster]

   3. (Geom.) The distance from a point in a curve to the chord;
      also, the versed sine of an arc; -- so called from its
      resemblance to an arrow resting on the bow and string.
      [Obs.]
      [1913 Webster]

   4. (Anat.) The larger of the two otoliths, or ear bones,
      found in most fishes.
      [1913 Webster]

   5. (Zool.) A genus of transparent, free-swimming marine worms
      having lateral and caudal fins, and capable of swimming
      rapidly. It is the type of the class Chaetognatha.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Sagitta
    n 1: a small constellation in the northern hemisphere between
         Cygnus and Aquila and crossed by the Milky Way
    2: any arrowworm of the genus Sagitta