The term “filament” is used in a number of different ways, with the meaning usually being clear from the context. Most of the uses of this word stay true to the Latin root, which means “to spin,” referencing the fact that filaments are long strings of material, which may be made from anything from wool to interstellar gases, depending on the filament in question.
Many people think of the filament in an incandescent lamp when they hear this word. Filaments in light bulbs, as they are also known, are made from metals which will emit bright light when they are heated with the use of an electrical current. To prevent the filament from combusting, the bulb is filled with an inert gas, although historically a vacuum was produced to achieve a similar effect. These filaments can produce light at varying degrees of intensity, and they will ultimately fail from stress, but they can endure for a surprisingly long time; one famous incandescent bulb in Livermore, California, was turned on in 1901 and was still burning in 2009.
The discovery of the filament was a critical step in developing electric light. While many people all over the world use light bulbs every day, there's a reason that a light bulb is sometimes used as a metaphor for a bright idea. Early pioneers not only had to develop the idea, but had to create a functional bulb which would avoid problems such as deposition of materials on the inside of the glass during operation, fires, poor quality of light, short life, and finding an appropriate material to use for the filament.
People also refer to filaments in biology. Part of the structure of the stamen, the male part of a flower, is known as the filament, and a filament can also be a long chain of proteins. Mycologists sometimes use the term when describing the hyphae, structures found in some fungi, because the hyphae can have a rather filamentous appearance. More generally, the term is sometimes used in the textile trade to refer to fine or thin yarns used in weaving, knitting, and other crafts.
This term also shows up in cosmology. Parts of the Sun's corona are sometimes referred to as filaments, and the term is also used to describe galaxy filaments, massive structures made up of linked galaxies. The identification of galaxy filaments has helped researchers learn more about the nature and structure of the universe.