(Or "host adaptor") A device that communicates between a computer and its SCSI peripherals. The SCSI adaptor is usually assigned SCSI ID 7. It is often a separate card that is connected to the computer's bus (e.g. PCI, ISA, PCMCIA) though increasinly, SCSI adaptors are built in to the motherboard. Apart from being cheaper, busses like PCI are too slow to keep up with the newer SCSI standards like Ultra SCSI and Ultra-Wide SCSI.
There are several varieties of SCSI (and their connectors) and an adaptor will not support them all.
The performance of SCSI devices is limited by the speed of the SCSI adaptor and its connection to the computer. An adaptor that plugs into a parallel port is unlikely to be as fast as one incorporated into a motherboard. Fast adaptors use DMA or bus mastering.
Some SCSI adaptors include a BIOS to allow PCs to boot from a SCSI hard disk, if their own BIOS supports it.
Adaptec make the majority of SCSI chipsets and many of the best-selling adaptors.
Note that it is not a "SCSI controller" - it does not control the devices, and "SCSI interface" is redundant - the "I" of "SCSI" stands for "interface".