byssi

noun

• An exceptionally fine and valuable fibre or cloth of ancient times. Originally used for fine flax and linens, the word was later extended to fine cottons, silks, and sea silk.
• The long fine silky filaments excreted by several mollusks (particularly Pinna nobilis) by which they attach themselves to the sea bed, and from which sea silk is manufactured.
• The stipe or stem of some fungi which are particularly thin and thread-like.