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Cap Screw
From WikiPedalia
A threaded fastener designed to screw into metal, with a head that is expected to stick up above the surface. Cap screws are commonly used on bicycles to secure bottle cages, luggage racks and other accessories. Cap screws are also used in many other locations on bicycles, but the nomenclature "cap screw" is rarely if ever used by practical mechanics. Most people call these "bolts" in practice, though sticklers may contend that is not proper usage.
See the Wikipedia article on Screws for more details on this.
