A version of the SCSI command specification.
SCSI-2 shares the original SCSI's asynchronous and synchronous modes and adds a "Fast SCSI" mode (<10MB/s) and "Wide SCSI" (16 bit, <20MB/s or rarely 32 bit).
Another major enhancement was the definition of command sets for different device classes. SCSI-1 was rather minimalistic in this respect which led to various incompatibilities especially for devices other than hard-disks. SCSI-2 addresses that problem. allowing scanners, hard disk drives, CD-ROM drives, tapes and many other devices to be connected.
Normal SCSI-2 equipment (not wide or differential) can be connected to a SCSI-1 bus and vice versa.