Ferrule is Latin for “small bracelet.” The word has many definitions, such as the band of metal that holds the eraser on the end of a pencil, a part on a golf club, a metal band used on the crown or root of a tooth, the tip on a walking stick or crutch, the white section of a pool cuestick that holds the padded tip, the metal band between the head and handle of a paintbrush, a component to align/protect a fiber optic cable, and so on.
In musical terms, a ferrule is a small band of metal used to keep wooden instruments from splitting, and also the band that holds the fibers on a violin or other bow. On string-through-body electric guitars like the Fender Telecaster, a ferrule is a small cup-shaped piece of metal with a hole in the end that secures the string in the back of the guitar body. Ferrules are also used with screws to secure the neck on some guitars and basses with carved neck heels where a neckplate won’t work.