From Diablo Wiki
Rerolling is a gaming term to describe starting fresh: making a new character, or picking a new class. The term comes from Dungeons and Dragons as well as other traditional pen and paper RPGs, where you would use dice to roll up statistics such as Strength, Charisma, etc., to create a new character. A lot of things about the game are random, and hence "reroll" is something the player would have to do if the character died.
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