A nut is a type of fastener with a threaded hole. Nuts are almost always used in conjunction with a mating bolt to fasten multiple parts together.

Bolts and nuts are used in several applications, with a primary function to hold things or components together. A bolt, also known as a screw, does not always have to be used together with a nut; however, a nut is always used together with a bolt.

Self-retained nuts provide a strong, permanent fastener for many types of thin materials; they are threaded blocks held in special enclosures attached to the part by welding, rivetting, screwing, or snap-on attachments. Single-thread nuts are formed by stamping a thread-engaging impression in a piece of flat metal.

A bolt or screw with a nut is widely used for fastening machine and structural components.