Multi-tap

A multi-tap is a peripheral that allows the player to hook up more controllers to a system than it has physical inputs.

It was originally designed by NEC for the PC Engine system, which only had one controller port. Since then, various companies have used with varying degrees of success; the most common may be the Sony PlayStation 2 multi-tap, since it was released with two controller ports in a generation where the other two systems both had four controller inputs.

Picture
http://www.playstationdirect.net/wp-content/uploads/2009/09/multitap.jpg