Bolts are a type of fastener. Fasteners are used to secure or fasten materials together. A bolt is a form of threaded fastener with an external male thread requiring a matching pre-formed female thread such as a nut.

Bolts use a wide variety of head designs, as do screws. These are designed to engage with the tool used to tighten them. Some bolt heads instead lock the bolt in place, so that it does not move and a tool is only needed for the nut end.

Bolts are often used to make a bolted joint. This is a combination of the nut applying an axial clamping force and also the shank of the bolt acting as a dowel, pinning the joint against sideways shear forces.