Immortality

Immortality (or eternal life) is the concept of living in a physical form for an infinite or inconceivably vast length of time. Immortality is one of the known limits to magic; it is impossible to make oneself immortal unless one makes use of a mystical object of great power to sustain life (such as the Philosopher's Stone created by Nicolas Flamel or a Horcrux, the latter having been used by Lord Voldemort). If one were to possess the three Deathly Hallows, it is fabled that they would possess the tools to become the "master of death". However, being a true "master of death" is to be willing to accept that Death is inevitable.

Philosopher's Stone
The Philosopher's Stone, a Stone created be famous alchemist Nicolas Flamel, is able to produce the Elixir of Life, one of the known means of immortality. In 1991 and 1992, a weakened Lord Voldemort tried to gain possession of the Stone so he could rise again after his downfall eleven years prior. The Stone was then destroyed by Albus Dumbledore and Nicolas Flamel himself in order to prevent this from happening again. With the distruction of the Philosopher's Stone, all individuals who were immortal because they drank the Elixir of Life (like Flamel and his wife) died after the supply of Elixir ran out.

Horcruxes
An Horcrux is an object chose to the purpose of being a receptacle of one's part of his soul, split by doing the most inhumane action: murder. If all the Horcruxes (and by extention the wizard's soul) are intact, the wizard is conidered immortal. Lord Voldemort split his soul several times in order to mantain his status of immortal being. His best plans were, however, beaten when Albus Dumbledore, Harry Potter, Ronald Weasley and Hermione Granger found each one of his Horcruxes and destroyed them, causing Voldemort to be able to die again.