An interrupt is said to be masked when it has been disabled, or when the CPU has been instructed to ignore it. A non-maskable interrupt (NMI) cannot be ignored, and is generally used only for critical hardware errors.
NMIs are normally delivered over a separate interrupt line. When an NMI is received by the CPU, it indicates that a critical error has occurred, and that the system is probably about to crash. The NMI is generally the best indication of what might have caused the problem.
Because NMIs are not able to be ignored, they are also used by some systems as a hardware monitor. The device sends a stream of NMIs, which are checked by an NMI handler in the processor. If certain conditions are met - such as an interrupt not being triggered after a specified length of time - the NMI handler can produce a warning and debugging information about the problem. This helps to identify and prevent system lockups.