What is the meaning of Permadeath?

Permanent death, where the player cannot continue but is obliged to restart the game from the beginning.

Likewise, any situation where a player character or characters who die are lost permanently but do not trigger a game over, allowing the player to proceed without the character(s). Particularly applies if said deaths are avoidable (not forced by the plot) and occur only as a result of player blunder.

Source: wiktionary.org