- a very fine thread or threadlike structure; a fiber or fibril: filaments of gold.
- a single fibril of natural or synthetic textile fiber, of indefinite length, sometimes several miles long.
- a long slender cell or series of attached cells, as in some algae and fungi.
- the stalklike portion of a stamen, supporting the anther.
- the barb of a down feather.
- (in a light bulb or other incandescent lamp) the threadlike conductor, often of tungsten, in the bulb that is heated to incandescence by the passage of current.
- the heating element (sometimes also acting as a cathode) of a vacuum tube, resembling the filament in an incandescent bulb.
- a solar prominence, as viewed within the sun's limb.
- the thin wire, usually tungsten, inside a light bulb that emits light when heated to incandescence by an electric current
- a high-resistance wire or ribbon, forming the cathode in some valves
- a single strand of a natural or synthetic fibre; fibril
- botany
- the stalk of a stamen
- any of the long slender chains of cells into which some algae and fungi are divided
- the barb of a down feather
- any slender structure or part, such as the tail of a spermatozoon; filum
- astronomy
- a long structure of relatively cool material in the solar corona
- a long large-scale cluster of galaxies
