A Jonline is a short line used by SCUBA-divers to fasten themselves to something. For example, it is used to fasten a diver to an anchor line during decompression stops in current.
The line is typically around 1 m (3 feet) long and equipped with a clip at each end. One clip is fastened to the diver's harness, and the other is used to fasten the line to the anchor line. In current this relieves the diver from holding on to the anchor line during the decompression stop.
The jonline is named after Jon Hulbert, who is credited with its invention.
A jonline is also often used to tether the diver's equipment to the dive boat before or after the dive. This enables the diver to get into or out of his or her equipment while in the water.
References
Jonline Wikipedia(Text) CC BY-SA