HMI stands for "Human-Machine-Interface" and contains a display and usually also a touch panel. The display of an HMI visualizes process parameters or status displays of a piece of equipment for the user or the service technician. Using the touch panel, entries can also be made directly on the HMI display.
Even the display of an MP-3 player together with individual keys could already be evaluated as a control unit or HMI. So there are also small HMI displays, with ELECTRONIC ASSEMBLY from 2" screen. These small displays are completely sufficient for many applications of smaller devices (HMI example EA uniTFTs020 with 2" incl. PCAP touch panel).
Essential for an easy to use HMI is a brilliant, bright display (possibly as IPS) and a fast, highly integrated intelligence. This enables intuitive user interfaces which are accepted with pleasure by the user.