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 .]
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.]
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.
3. (Bot.) An obsolete name for certain fungi composed of