Jig

A jig is a fixture -- shop-built or purchased -- that holds a workpiece or guides a tool through a specific cut or operation, trading a bit of setup time for accuracy and repeatability that would be hard or risky to achieve freehand. A box joint indexing jig, a doweling jig, a crosscut sled: all solve the same basic problem, turning a cut that would otherwise depend on a steady hand into one that depends on an accurate setup instead.