Spider silk, also known as gossamer, is a protein fiber spun by spiders. Spiders use their silk to make webs or other structures, which function as nets to catch other creatures, or as nests or cocoons for protection for their offspring. They can also suspend themselves using their silk, normally for the same reasons.
Many small spiders use silk threads for ballooning, the scientific term for the dynamic kiting spiderlings (mostly) use for dispersal. They extrude several threads into the air and let themselves become carried away with upward winds. Although most rides will end a few meters later, it seems to be a common way for spiders to invade islands. Many sailors have reported that spiders have been caught in their ship's sails, even when far from land.
In some cases, spiders may even use silk as a source of food.
Peasants in the southern Carpathian Mountains used to cut up tubes built by Atypus and cover wounds with the inner lining. It reportedly facilitated healing, and even connected with the skin. This is believed to be due to antiseptic properties of spider silk.
Some fishermen in the indo-pacific ocean use the web of Nephila to catch small fish.
The silk of Nephila clavipes has recently been used to help in mammalian neuronal regeneration.
At one time, it was common to use spider silk as a thread for crosshairs in telescopes, microscopes and similar optical instruments.