The Collaborative International Dictionary of English v.0.48:

Pinnule \Pin"nule\, n. [L. pinnula, dim. of pinna feather: cf.
   F. pinnule.]
   1. (Bot.) One of the small divisions of a decompound frond or
      leaf. See Illust. of Bipinnate leaf, under Bipinnate.
      [1913 Webster]

   2. (Zool.) Any one of a series of small, slender organs, or
      parts, when arranged in rows so as to have a plumelike
      appearance; as, a pinnule of a gorgonia; the pinnules of a
      crinoid.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pinnule
    n 1: division of a usually pinnately divided leaf [syn: pinna,
         pinnule]