Azkaban

"They don't need walls and water to keep the prisoners in, not when they're trapped inside their own heads, incapable of a single cheerful thought. Most go mad within weeks."

- Remus Lupin on Azkaban

Azkaban is a prison located on an island in the middle of the North Sea that serves the magical community of Great Britain. Using certain Charms, the prison is hidden from the muggle world, and is unplottable. It is thought to have undetectable extension charms placed on it (or similar types of magic) to make it bigger on the inside, as the prison seems to serve the whole of wizarding Britain. Although other wizarding prisons exist, such as Nurmengard (which was used exclusively to hold its creator Gellert Grindelwald), Azkaban is the only known fortress to be the "official" holding ground for wizarding criminals.

Azkaban gained a reputation as a horrible place, mostly due to the prison's guards: Dementors, foul creatures that suck the happiness out of the victim and cause great suffering to those in their proximity. Because of this, most of the prisoners eventually went insane and slowly died under the Dementors' influence. Since 1717, using any of the three Unforgivable Curses on another human being has carried a punishment of a life sentence in Azkaban (unless there is sufficient evidence that the caster did so under the influence of the Imperius Curse, or legal exceptions made by the Ministry of Magic as in the case of Aurors in the First Wizarding War).

Pre-1998
Azkaban was guarded by Dementors until most of them revolted and joined Lord Voldemort in summer of 1995. As a result, Voldemort's Death Eaters escaped en masse to rejoin their master in both 1996 and 1997.

There is most likely an Anti-Apparation jinx on Azkaban; if there were not, wizards and witches could simply Apparate from the prison to the outside world. However, since it is debatable that most witches and wizards need a wand to Apparate, this may be irrelevant, as none of the prisoners maintain possession of their wands. Before 1995, most of the prisoners would barely have had enough strength to Apparate anyway, as the mere presence of Dementors, particularly in large numbers, inhibits the use of magical powers.

Prior to the Dementors' revolt, the only people ever to escape the prison were Barty Crouch Jr and Sirius Black, in 1982 and 1993, respectively. Barty Crouch Jr. was smuggled out by his father, Barty Crouch Sr., and replaced by his dying mother using Polyjuice Potion, while Sirius was able to escape by changing into his Animagus form of a dog. The revolt has severely compromised the security, as multiple escapes were made with no more Dementors guarding the prison; the Ministry continued omitting certain facts to maintain their facade of having everything under control, including keeping one breakout a secret.

There is a graveyard outside the wall of Azkaban where the Dementors buried prisoners who died. However, if the dead prisoner has a relative who is not a criminal, the relative in question has the right to pick up the corpse for a proper burial, as Barty Crouch Sr. had the right to pick up the body that was disguised as his son, though he chose not to due to fear of being exposed of committing a jailbreak.

Although the prison is set in the middle of the ocean with iron walls protecting it, it was unnecessary to keep prisoners inside, as Remus Lupin stated, because the prisoners were already trapped in their own heads due to the intense depression caused by the Dementors. As Sirius indicated, most prisoners went insane after a short while, and some even stopped eating, preferring death to their lives within Azkaban. Dementors fed the prisoners, indicating the prisoners might have felt depression at mealtimes.

Visits to the island is heavily restricted, as only respectable Ministry officials were noted to be allowed to visit any possible relatives. An example is when Barty Crouch Sr. and his wife were allowed a deathbed visit of their imprisoned son due to the former's high-status in the government; they took advantage of this privilege to smuggle Barty Jr. out of prison. The Minister of Magic is also allowed to visit the prison, as to check out on certain prisoners for their conditions, as Cornelius Fudge did on Sirius Black.

It is not known whether wizards made the Azkaban island magically, or whether it was an existing ordinary rock island which wizards took over. If the latter is the case, when the sea level was lower in the it would have been a rock hill standing above  which is now the bed of the North Sea.

After the war
After the end of the Second Wizarding War and Kingsley Shacklebolt's appointment as Minister for Magic in 1998, dementors were no longer employed as guards of Azkaban. This is likely because they had proven to be untrustworthy, and because the depression and madness their presence caused in prisoners is considered inhumane. Presumably, other security measures were employed to guard the prisoners, most likely far more humane. It is unknown how many surviving Voldemort supporters and Death Eaters were imprisoned in the aftermath. Them aside, any Ministry officials who knowingly and willingly went with Voldemort's ideologies during the time when the Ministry was under his control, mainly the Muggle-Born Registration Commission, were sentenced to Azkaban, the most prominent being Dolores Umbridge, for her crimes against humanity (more specifically, Muggle-borns).

Known Azkaban convicts
"We have confirmed that ten high security prisoners, in the early hours of yesterday evening, did escape. And of course, the Muggle Prime Minister has been alerted to the danger. We strongly suspect that the breakout was engineered by a man with personal experience in escaping from Azkaban; notorious mass murderer Sirius Black, cousin of the escapee Bellatrix Lestrange."

- Cornelius Fudge interview to the Daily Prophet about the Azkaban Mass Breakout of January 1996

Etymology
The name Azkaban is inspired by the Russian town Abakan, which has historically been occupied by various groups of exiles and also draws inspiration from the Prison of Alcatraz off the coast of San Francisco.

Behind the scenes

 * There is an added scene in the film adaptation of Harry Potter and the Order of the Phoenix in which the 1996 mass breakout at Azkaban is shown.
 * In the film, Azkaban is also on an island, but the island cannot be seen due to the stormy sea.
 * In the front cover of the adult Bloomsbury edition of the third book, Azkaban is shown to be a rectangular tower, while in the fifth film it is more of a triangular tower.
 * It is likely that if Sirius's status as an Animagus was known, the Ministry would have taken certain precautions to prevent him from transforming to escape. This would mean that Azkaban's security measure is not fully enforced for every situation at all times.
 * Escaping from Azkaban is apparently a very serious crime, punishable by the worse-than-death Dementor's Kiss. Sirius Black was fated to this, if not for him being rescued beforehand, and Bartemius Crouch Jr. suffered from it when his escape became public. Such a punishment is probably because the escaped prisoners in question are too dangerous to be simply reconfined into the prison, as they could easily escape again.
 * Prisoners from Azkaban have their place card numbers tattoed on their neck.

Appearances

 * Harry Potter and the Chamber of Secrets
 * Harry Potter and the Chamber of Secrets (film)
 * Harry Potter and the Chamber of Secrets (video game)
 * 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 Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film) 
 * Harry Potter and the Order of the Phoenix (video game)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 2 (video game)
 * LEGO Harry Potter: Building the Magical World
 * LEGO Harry Potter: Characters of the Magical World
 * LEGO Harry Potter: Years 1-4
 * Pottermore