Final answer:
The MDI key in CNC machines stands for Manual Data Input and allows operators to manually input commands into the machine.
Step-by-step explanation:
The MDI key, in CNC machines such as Haas, stands for Manual Data Input. The MDI key allows the operator to manually input commands into the machine by using the machine's control panel. This feature is commonly used for setting up the machine, troubleshooting, or performing manual operations.