The Display, Keyboard and Mouse, that I use to program and monitor our SCADA system, is the HMI.
The 3 different synced computers, running the Master Program, all the comm paths, the Remote Terminal Units (RTU), in the field, and the HMI, are all part of the SCADA system. A SCADA system needs to have an HMI, while an HMI doesn't have to be part of a SCADA system.