Wire nut

A wire nut is a cone-shaped cap, usually plastic, that twists over stripped wire ends to hold them together and cover the joint. It is a common way to make a splice inside a junction box. A loose connection can overheat, so the right type and a firm joint matter.

OSHA's wiring rule says conductors shall be spliced or joined with splicing devices identified for the use. CPSC also warns that twist-on connectors are an inadequate fix for aluminum wiring. Boxes should be opened and worked on only by a licensed electrician. See residential electrician.