Often refers to the 4-pin connectors used to attach DC power to the drives inside a PC cabinet. Molex is a large manufacturer of electronics plugs and sockets that dates back to the 1940s. Since it was one of the first companies to make these connectors, the Molex name has stuck, even though they are also made by others.
Power Supply Connectors
These "Molex" connectors bring DC power to the drives inside a PC case. The large one on the right is used for disk, CD-ROM and DVD drives, while the small connector is used for floppy drives and other devices.