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):

byssus
    n 1: tuft of strong filaments by which e.g. a mussel makes
         itself fast to a fixed surface [syn: byssus, beard]