A piece of metal, usually square or hexagonal in shape, with a hole through it having machined internal threads, intended to be screwed onto a bolt or other threaded shaft.
On stringed instruments such as guitars and violins, the small piece at the peghead end of the fingerboard that holds the strings at the proper spacing and, in most cases, the proper height.
A shaped piece of metal, threaded by a wire loop, which is jammed in a crack in the rockface and used to protect a climb. (Originally, machine nuts [sense #2] were used for this purpose.)