VHDCI is a type of connector often found on computers. It's officially part of the SCSI-3 standard and is usually used for SCSI devices. The size of the connector, about 2mmx35mm, allows up to four VHDCI connectors on a single PCI or PCI Express card.
In addition to SCSI, VHDCI is also used for some uncommon video devices, most particularly the video I/O board on the Silicon Graphics O2.