The Collaborative International Dictionary of English v.0.48:

Pinna \Pin"na\, n.; pl. Pinn[ae], E. Pinnas. [L., a
   feather.]
   1. (Bot.)
      (a) A leaflet of a pinnate leaf. See Illust. of Bipinnate
          leaf, under Bipinnate.
      (b) One of the primary divisions of a decompound leaf.
          [1913 Webster]

   2. (Zool.) One of the divisions of a pinnate part or organ.
      [1913 Webster]

   3. [L. pinna, akin to Gr. ?.] (Zool.) Any species of Pinna,
      a genus of large bivalve mollusks found in all warm seas.
      The byssus consists of a large number of long, silky
      fibers, which have been used in manufacturing woven
      fabrics, as a curiosity.
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   4. (Anat.) The auricle of the ear. See Ear.
      [1913 Webster]