The Collaborative International Dictionary of English v.0.48:

Ligule \Lig"ule\ (l[i^]g"[-u]l), n. [L. ligula, lingula, little
   tongue, dim of lingua tongue : cf. F. ligule.]
   1. (Bot.)
      (a) The thin and scarious projection from the upper end of
          the sheath of a leaf of grass.
      (b) A strap-shaped corolla of flowers of Composit[ae].
          [1913 Webster]

   2. (Anat.) A band of white matter in the wall of fourth
      ventricle of the brain.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ligule
    n 1: (botany) any appendage to a plant that is shaped like a
         strap