Life debt

A life debt is a magical bond formed between a wizard or witch and the person whose life they saved. The one who owes the debt to the saviour would one day be obliged to repay the deed by doing something beneficial to the said saviour. Such a bond can be formed between even the worst of enemies and regardless of whether either of the involved parties want it or not. As this bond is magically binding, the indebted could commit the repayment without acknowledging it, or even against their will.

Known life debts

 * James Potter, in his fifth year, saved Severus Snape from a near-fatal prank that Sirius Black played on him, which involved luring Snape to the Shrieking Shack where Remus Lupin was in his werewolf form. As such, Snape owed James a life debt, although they hated each other. This was repaid when Snape repeatedly protected James' son, Harry, throughout Harry's years at Hogwarts School of Witchcraft and Wizardry. However, it is possible that Snape chose to protect Harry solely because of his love of Harry's mother, Lily Potter.
 * Harry Potter, in his third year, protected Peter Pettigrew from being murdered by his former best friends, Sirius Black and Remus Lupin, for treason. Instead, Harry asked for him to be transferred to Azkaban. Though Pettigrew escaped, he still owed Harry a life debt. This was repaid when Pettigrew showed a rare and brief moment of mercy at the Skirmish at Malfoy Manor and released Harry from a choking hold with his silver hand, which Voldemort had given to him for sacrificing his own hand to bring Voldemort back into human form. Unfortunately, it was fulfilling this debt that caused the silver hand to then turn on Peter, killing him for showing what Voldemort would have considered weakness.

Behind the scenes

 * Both known life debts are related to Harry's benefit, and both the in-debt people died protecting him.
 * J.K. Rowling has not commented on whether Arthur Ginny, or Ron ever had life debts to Harry, for being rescued from being bitten by Nagini, the Department of Mysteries, rescued in the Chamber of Secrets, and given a bezoar when poisoned, respectively. If it is true that Ron once owed Harry a life debt, he presumably repaid it by saving Harry from drowning in Deathly Hallows.
 * It is unknown if life debts can be made between animals and people, but if so Harry is in debt to Fawkes the Phoenix for saving him from the Basilisk venom, and Dumbledore was in debt to Fawkes for taking the Killing Curse for him during his duel with Voldemort.

Appearances

 * Harry Potter and the Philosopher's Stone
 * Harry Potter and the Philosopher's Stone (film)
 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (film)
 * Harry Potter and the Prisoner of Azkaban (video game)
 * Harry Potter and the Deathly Hallows

Notes and references
Dette de vie Hengenvelka