Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. (Zo["o]l.) One of the divisions of a pinnate part or
      organ.

   3. [L. pinna, akin to Gr. ?.] (Zo["o]l.) 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.

   4. (Anat.) The auricle of the ear. See {Ear}.

Source : WordNet®

pinna
     n 1: division of a usually pinnately divided leaf [syn: {pinnule}]
     2: the externally visible cartilaginous structure of the
        external ear [syn: {auricle}, {ear}]
     [also: {pinnae} (pl)]

pinnae
     See {pinna}