Bolt-on neck
Bolt-on neck is a method of guitar (or similar stringed instrument) construction that involves joining a guitar neck and body using screws or bolts, as opposed to glue as with set-in neck joints.
Methods
The "bolt-on" method is used frequently on solid body electric guitars and on acoustic flattop guitars. In the typical electric guitar bolt-on neck joint, the body and neck cross in horizontal plane, the neck is inserted in a pre-routed "pocket" in the body, and they are joined using 3 or 4 (rarely 6) screws.
As screw heads damage the wood and could put extra stress on it, typically a rectangular metal plate or a pair of metal plates are used to secure the joint and re-distribute the screw pressure evenly. Such a plate is usually criticized for making playing on top frets uncomfortable, so, manufacturers sometimes employ some kind of more intricate method to hide a metal plate, smooth the angles and make access to top frets easier. However, a visible metal plate is usually considered as a part of "vintage" style and they are a popular place to emboss manufacturer's logos, stamp out serial numbers and put other artwork.