Definition of: byssus

(bis′əs) noun plural bys·sus·es or bys·si (bis′ī)
1. Formerly, any costly white stuff of cotton, silk, or linen.
2. Zool. A bunch of silky thread secreted by the foot of certain stationary bivalve mollusks, as mussels, and serving as a means of attachment to an anchorage.
3. Bot. A filamentous fungus of the obsolete group Byssi. [<L <Gk. byssos fine linen]