Undeath is a situation that can occur in N in some of the more devious levels. Basically, undeath is when the ninja is unable to go anywhere or solve the level in any way. This can happen if the ninja is trapped in a cell with a trap door blocking the only way in, a trap door blocks the way to the exit, or an enemy such as a zap drone or a floorguard gets into a position where it blocks the ninja. If you get into a situation with undeath, your only option is to kill yourself and do the level again.
Also, if you hit a launchpad and either a mine or zap drone in the same frame, you will survive but be unable to interact with anything except tiles and closed doors. In debug mode, you will still be able to interact as normal.