Small Computer System Interface (SCSI) is a group of ANSI electronic interfaces that allow personal computers (PCs) to establish communication with peripheral hardware, including tape drives, disk drives, printers, CD-ROMs, scanners and so on. Communication using SCSI is quicker and more flexible than earlier parallel data transfer interfaces.
SCSI is pronounced as "SKUZ-ee."
Techopedia explains SCSI
SCSI, first completed in 1982, is one of the most frequently used interfaces for disk drives. In 1986, ANSI adopted the initial version, SCSI 1. It was an 8-bit version and had a transfer speed of 5 MBps, which permitted connection with as many as eight devices using a cable having a maximum length of up to 6m.
The latest SCSI version is the 16-bit Ultra-640 (Fast-320). This version has been available since 2003, with a transfer speed of 640 Mbps. It allows connecting as many as 16 devices using a cable having a maximum length of up to 12m.
SCSI-2 or later has the ability to support a maximum of seven peripheral devices, including CD-ROMs, hard drives and scanners. These peripheral devices can be attached to a single SCSI port on a system bus. SCSI ports were originally built for Unix computers and Apple Macintosh; however, they can be used with PCs as well.
Even though not all devices are able to support all SCSI levels, SCSI standards are usually backward-compatible. In personal computing, the Universal Serial Bus (USB) has replaced SCSI interfaces in most cases. However, enterprise-level customers still make use of SCSI inserver farms for hard disk controllers.
Serial-attached SCSI (SAS) solutions work with devices that implemented previous SCSI technologies. If the SCSI performance is not sufficient, the Serial Storage Architecture (SSA) standard can be used, which maintains the SCSI command set by embedding SCSI-3 over Transmission Control Protocol/Internet Protocol (TCP/IP).