Microsoft Knowledge Base Article 101352 begins with:
A terminator is a set of resistors placed at each end of a transmission line, such as the SCSI (small systems computer interface) bus. Terminators help to prevent signal reflections at the ends of the transmission line and ensures that the signal on the line quickly reaches its desired state. A correctly terminated SCSI bus supports faster and more reliable operation. Each end of the physical SCSI bus should have one SCSI terminator attached.