Death

"It is not about striving for immortality, but about accepting mortality."

- J. K. Rowling

Death is the end of a living organism's life, technically defined in humans as either the permanent termination of heart function and respiration, or of brain activity.

Afterlife
"After all, to the well-organised mind, death is but the next great adventure."

- Albus Dumbledore comments on the afterlife

While Death is understood in a purely physical sense, the nature of what lies beyond it is a mystery to wizards and witches and Muggles alike. When a wizard or witch dies, unlike a Muggle, he or she can choose to leave behind an imprint of their soul in the mortal world in the form of a ghost. Few opt to become ghosts, however, as it means they will never "go on" like most people do. Those who cross over to the other side cannot come back in any form resembling the physical body they had in life. The exception being the beings brought back using the Resurrection Stone. Though, as it is described in the seventh book, they do not belong in the mortal world, being only a semi-corporeal form, "less substantial" than a living body but "much more" than a ghost.[6]

Immortality and resurrection
There is currently no known way to magically reunite a person's soul with their body once they have died. The Resurrection Stone can recall someone who has died from the beyond, but they will return only in a semi-corporeal form, "less substantial" than a living body but "much more" than a ghost. Phoenixes are the sole exception to the rules of death, as they can be reborn from their ashes without any restraints or assistance.

However, while there exists no known method of reversing death once it has occurred, there are certain things a witch or wizard can do to postpone their death or prolong their life (even further than the longevity which would seem to be granted by magical ability e.g. Albus Dumbledore's health despite his advanced age).

The Elixir of Life, which is made from the Philosopher's Stone, will grant a person extended life for as long as they continue to consume it. However, because the only known Philosopher's Stone was destroyed in 1992, this method is no longer available. Unicorn blood can keep alive a person who is near death, but unicorns are such pure, defenceless creatures that a person who kills one and drinks its blood will have "but a half-life." A wizard or witch who rips their soul through an act of murder can place that torn fragment inside of an external object called a Horcrux. By binding a part of their soul to the earth, the Horcrux prevents the wizard or witch from dying, even if their body is injured or completely destroyed. However, there is a cost to using Horcruxes — as shown in the deterioration of Lord Voldemort's physical condition after repeatedly splitting his soul, as well as the mangled spectral state he has been trapped in during the destruction of his physical body. There is a potion which enables the Horcrux-creator's body to be reconstructed in the latter scenario. Overall, however, many wizards and witches would prefer death over such a pitiful state of existence.

Corpses can also be reanimated through Dark magic. Known as Inferi, these creatures are not alive, and simply do whatever the wizard who controls them wants, like puppets.

Study and perception of death
There is a chamber in the Department of Mysteries where witches and wizards study the mysteries of death. In this chamber is the Veil, an ancient stone archway, which is a gateway between the world of the living and the world of the dead. People standing around the Veil may hear voices from the other side depending on their level of faith in an afterlife. A person whose body passes through the Veil will die.

Wizarding philosopher Bertrand de Pensées-Profondes also researched death. He wrote a highly-regarded work on the subject, A Study into the Possibility of Reversing the Actual and Metaphysical Effects of Natural Death, with Particular Regard to the Reintegration of Essence and Matter.

Thestrals are magical creatures visible only to people who have witnessed a death firsthand. However, the ability to see thestrals does not come immediately, but only after one has had time to fully understand death and its finality.