An Etymological Dictionary of Astronomy and Astrophysics

1. A long tongue of a relatively cold matter (10,000 K), suspended in the → solar corona (2 million K). Filaments seem dark in the Hα light when they are seen projected on the solar disk; at the limb they look as what they actually are: → prominences.
   

2. Electricity, Electronics: In electric light bulbs and thermionic valves, a wire of metal of high melting point, which is heated by the passage of an electric current.
   

3. Cosmology: → cosmic filament.

Etymology (EN): From Mod.L. filamentum, from L.L. filare “to spin, draw out in a long line,” from L. filum “thread,” from PIE base *g^whi- “thread, tendon;” cf. Gk. bios “bow;” Skt. jiyā- “bow-string;” Av. jiiā- “bow-string;” Mod.Pers. zeh “string, bow-string” (dialectal Qomi zij “mason’s cord”);
Arm. jil “string, line;” Lith. gijà “thread;” Russ. žica “thread.”

Etymology (PE): Rešté “thread, line, file,” from reštan, ristan “to spin;” Mid.Pers. ‘rws- “to turn to,” abrešom “silk;” Sogdian rwyšt “spun;” Av. uruuaēs- “to twist, turn;” Proto-Iranian *uris- “to turn, spin.”