A device that holds a piece of machine work and guides the tools operating on it WordNet 3.0 & Open English WordNet

A jig is a type of custom-made tool used to control the location and/or motion of parts or other tools. Wikipedia

A device/frame used to control position or motion Wikipedia Disambiguation

Custom-made tool used to control the location and/or motion of another tool Wikidata

A device in manufacturing, woodworking, or other creative endeavors for controlling the location, path of movement, or both of either a workpiece or the tool that is operating upon it. Subsets of this general class include machining jigs, woodworking jigs, welders' jigs, jewelers' jigs, and many others. Wiktionary