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.

The shop-made ones have a way of taking over a Saturday that was supposed to be spent on the actual project. That's not wasted time, exactly: building the thing that builds the things is most of how a shop gets good. It's just worth noticing when the jig has become the hobby.