Bolt, mechanical fastener that is usually used with a nut for connecting two or more parts. A bolted joint can be readily disassembled and reassembled; for this reason bolts or screw fasteners are used to a greater extent than any other type of mechanical fastener and have played an important part in the development of mass-produced articles and steel structures. The bolt consists of a head and a cylindrical body with screw threads along a portion of its length. The nut is the female member of the pair, having internal threads to match those of the bolt. Washers are often used to prevent loosening and crushing.