Loosely speaking, a disk format sometimes used by MS-Windows, Mac OS, AmigaOS, and (almost always) UNIX. Generally SCSI is superior (but more expensive) to IDE, but it varies somewhat with system load and the individual SCSI and IDE components themselves. The quick rundown is that: SCSI-I and SCSI-II will almost always outperform IDE; EIDE will almost always outperform SCSI-I and SCSI-II; SCSI-III and UltraSCSI will almost always outperform EIDE; and heavy system loads give an advantage to SCSI. Note that although loosely speaking it is just a format difference, it is deep down a hardware difference.