SCSI(Small Computer System Interface) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols and electrical and optical interfaces. SCSI is most commonly used for hard disk and tape drives, but it can connect a Wide range of other devices, including scanners and CD drives.
The SCSI standard defines command sets for specific peripheral device types but the standard is highly pragmatic and addressed toward commercial requirements.