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Permadeath, short for permanent death, is a feature in video games where if a character dies, they will stay dead for the rest of the game. The only way be able to play dead characters would be to load the game file before the character died or to start the game all over again.
This is a big feature in Fire Emblem games. Permadeath adds a level of challenge to the player, so the player must be very careful where they move each of their own player units. However, in every Fire Emblem game since Fire Emblem: Awakening, the player has the choice whether or not they want to play the game with permadeath.