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.
      [1913 Webster]

   4. (Anat.) The auricle of the ear. See Ear.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Byssus \Bys"sus\, n.; pl. E. Byssuses; L. Byssi.[L. byssus
   fine flax, fine linen or cotton, Gr. by`ssos .]
   [1913 Webster]
   1. A cloth of exceedingly fine texture, used by the ancients.
      It is disputed whether it was of cotton, linen, or silk.
      [Written also byss and byssin.]
      [1913 Webster]

   2. (Zool.) A tuft of long, tough filaments which are formed
      in a groove of the foot, and issue from between the valves
      of certain bivalve mollusks, as the Pinna and Mytilus,
      by which they attach themselves to rocks, etc.
      [1913 Webster]

   3. (Bot.) An obsolete name for certain fungi composed of
      slender threads.
      [1913 Webster]

   4. Asbestus.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pinna
    n 1: division of a usually pinnately divided leaf [syn: pinna,
         pinnule]
    2: the externally visible cartilaginous structure of the
       external ear [syn: auricle, pinna, ear]