A filament fiber is a single strand, indefinite in length, of natural or synthetic fiber such as cotton or nylon that can be spun or woven. There are also variable definitions, according to Dictionary.com
While most often describing thread that is used for textiles, in general "filament fiber" can refer to any threadlike structure. For instance, in science the term refers to a long slender cell or series of attached cells. The filament fiber in a light bulb or other incandescent lamp is the threadlike conductor that produces light when heated by the passage of current. Similarly, in electronics a filament fiber refers to the heating element of a vacuum tube, which resembles the filament in a light bulb.