Short for "permanent death". it is one of the main feature of Roguelike.
The authors of Rogue mentioned that Permadeath is meant to be more about "consequence persistence", and it is a mechanic that makes games immersive by making things matter. see Ken Arnold and Michael Toy and Glenn Wichman: Rogue Panel(Arnold, Toy, and Wichman 2016).
Reference
Arnold, Ken, Michael Toy, and Glenn Wichman. 2016. “Rogue Panel.” Roguelike Celebration. September 17, 2016. https://www.youtube.com/watch?v=4IkrZkUV01I.