@ErisedKT Hi, on Ebay it is called a 5.5 * 2.1 DC female screw terminal connector, you can also buy the male version. They are very handy when using bench power supplies or battery packs to power circuit boards with female barrel jack connectors built in. You have to be very careful to wire them correctly, usually, but not always, the centre pin is positive. The female versions are also useful where you have a 'wall wart' power supply and want to power a circuit board without a barrel jack connector. 5.5 mm is the outside diameter and 2.1 is the inside diameter of the connector. Always check the polarity of the device you are connecting to.