#It was the Elder Wand, Harry’s blood, and Harry’s choice.

J.K. Rowling gives a detailed answer to this on the FAQ section of her old website, and attributed Harry’s survival to three different factors - Lily’s blood in the Dark Lord, Harry being the true master of the Elder Wand, and the choices both Harry and the Dark Lord made.

###Lily’s sacrifice protection

The first part was that the Dark Lord had Harry’s blood in him and tethered him to life by keeping the enchantment alive.

Again, Voldemort violated deep laws of magic he did not understand, but there is more to it than that.

Having taken Harry’s blood into himself, Voldemort is keeping alive Lily’s protective power over Harry. So Voldemort himself acts almost like a Horcrux for Harry – except that the power of Lily’s sacrifice is a positive force that not only continues to tether Harry to life, but gives Voldemort himself one last chance (Dumbledore refers to this last hope in chapter 35). Voldemort has unwittingly put a few drops of goodness back inside himself; if he had repented, he could have been healed more deeply than anyone would have supposed. But, of course, he refused to feel remorse.
_{^{- FAQ on J.K. Rowling’s website}}

Dumbledore expected this to happen, and explained it to Harry.

“He took your blood believing it would strengthen him. He took into his body a tiny part of the enchantment your mother laid upon you when she died for you. His body keeps her sacrifice alive, and while that enchantment survives, so do you and so does Voldemort’s one last hope for himself.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 35 (Kings Cross)}}

###Harry being the true master of the Elder Wand

The second part is that the Dark Lord used the Elder Wand, a wand that saw Harry as its master, againt Harry, so none of the spells he tried to cast on Harry worked.

Voldemort is also using the Elder Wand - the wand that is really Harry’s. It does not work properly against its true owner; no curse Voldemort casts on Harry functions properly; neither the Cruciatus curse nor the Killing Curse.

The Avada Kedavra curse, however, is so powerful that it does hurt Harry, and also succeeds in killing the part of him that is not truly him, in other words, the fragment of Voldemort’s own soul still clinging to his. The curse also disables Harry severely enough that he could have succumbed to death if he had chosen that path (again, Dumbledore says he has a choice whether or not to wake up). But Harry does decide to struggle back to consciousness, capitalises on Lily’s ‘escape route’, and pulls himself back to the realm of the living.
_{^{- FAQ on J.K. Rowling’s website}}

###The choices both Harry and the Dark Lord made

However, the choices that both Harry and the Dark Lord had made had a large part in the final outcome of events. If either of them had made even one different choice, the outcome may have been different.

It is important to state that I always saw these kinds of magic (the very deepest life and death issues) as essentially un-scientific; in other words, there is no “Elder Wand + Lily’s Blood = Assured Survival” formula. What count, ultimately, are Harry and Voldemort’s own choices. They have each been given certain weapons and safeguards, but the power of these objects and past happenings lie in how they are understood, and how they are used or enacted upon. Harry has a deeper and truer understanding of the meaning of the objects and past events, but his greatest powers, those that save him, are free will, courage and moral certainty.
_{^{- FAQ on J.K. Rowling’s website}}

One of the simplest, though certainly not the only, example of this was that Harry could have chosen to die when the Killing Curse hit him.

##Harry being “master of death”, in context, didn’t seem literally meant.

Harry being a “master of death” is never mentioned as a reason for his survival, either in the books, or in interviews with J.K. Rowling. It was only ever mentioned once, by Dumbledore when he’s telling Harry he was wise to not fear death.

“You are the true master of death, because the true master does not seek to run away from Death. He accepts that he must die, and understands that there are far, far worse things in the living world than dying.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 35 (Kings Cross)}}

Used in this way, it seems to be more of a statement that it’s best to accept that death happens and Harry was honorable to actively choose to die for a noble cause, rather than a literal statement about him possessing all the Hallows. In fact, his possession of all three Hallows shortly before the confrontation is never mentioned as a factor in his survival.