An electronic component very similar in appearance to a resistor, but with a rating of nominally zero ohms.
The body of the component is the same as a small resistor, but instead of color coded bands, it has a single wide black band.
The function of a zero ohm link is the same as a jumper, in that it adds options to PCBs, so that one PCB can be built into a selection of similar products. However it is intended as a factory option and not intended to be changed by the user. Also, since it is soldered to the board, it is more reliable than a jumper.
The disadvantage of zero ohm links is that they are more costly than a simple piece of wire, or a PCB track. However, this cost is offset by the fact that the links can be automatically inserted to a board by the same machine which places resistors.
A final advantage is during troubleshooting. A zero ohm link is a visual cue that an option is available and a particular decision has been made for this particular board.