Noun
wire nut (plural wire nuts)
A hollow cone-shaped device with a conductive interior and an insulating exterior (usually plastic) applied over two (or more) wires (5mm or less) twisted together to make an electrical connection.
As wire nuts tend to come off of single wires, twist a short length of electrical tape around the wire nut and wire to keep them together. Source: Internet
Screw on a wire nut. Source: Internet