If this is a 260 thats the engine reset button.
That is an engine reset button in the US 260Z courtesy of the inane thinking of the federal government. To get people to buckle up, they required that the starter be interlocked with the seatbelts. However, if you put a bag of groceries in the passenger seat, you might have to fasten the passenger side seat belt to start the car. There was a button in the engine bay to work around this in emergencies. It's easier to jumper it out.