Black 5.0mm (approx. 3/16") Heat Shrink Tubing
This size Heat Shrink Tubing is commonly used for insulating electrical connectors such as 3.5mm bullet connectors. The tubing shrinks to about 3/32" in diameter, and can be shrunk with the use of a soldering iron, heat gun, or butane lighter.
The picture below is for reference only - no wire or connector included.