A bolt-on neck is a method of guitar construction that involves joining a guitar neck and body using screws as opposed to glue as with set-in neck joints. The term is a misnomer, introduced mostly by Fender whose guitars traditionally had "bolt-on necks".
This method is used frequently on solid body electric 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, regularly using 4 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.
With traditional bolt-on neck constructions, a substantial piece of wood on the body side is needed to house the screws and keeping the construction 'sturdy'. This neck 'heel' can be a real obstruction when trying to reach the upper frets. As an alternative to the thicker and more square "traditional" bolt-on neck heels Ibanez developed the Tilt Joint, Cutaway Heel and as the most slim alternative, the All Access Neck Joint.