Short for "permanent death". it is one of the main feature of Roguelike.
Berlin Interpretation
Berlin Interpretation(Yehieli, Dopieralski, and Lait 2008) specifically defines it as "You start over from the first level when you die".
Temple of the Roguelike Interpretation (2023)
What a roguelike is(Zapata 2005) defines it as "Permanent Consequences": the outcome of the player's actions cannot be rolled back by reloading.
Rogue Panel
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.
Yehieli, Ido, Radomir Dopieralski, and Jeff Lait. 2008. “Berlin Interpretation.” 2008. https://roguebasin.com/index.php/Berlin_Interpretation.
Zapata, Santiago. 2005. “What a Roguelike Is.” 2005. https://roguebasin.com/index.php/Roguelike.