An NMI is a Non Maskable Interrupt. It represents a serious condition which compromises the integrity of the system, prompting the OS to shut the system down.
On a lighter note, when I worked for a computer manufacturer, a colleague and I were standing at a traffic light having a chat, together with our technical manager. A sudden gust of wind blew up the skirt of a pretty young girl nearby, exposing her underwear and putting an immediate stop to our conversation.
Our manager remarked, "Now that's what you call a Non Maskable Interrupt".