Pure-blood

Pure-blood is the term for wizards and witches who claim to be from families with purely magical heritage. This means that there are no Muggles in their family trees, but this is rarely true, for if wizards didn't mate with Muggles, their race would die out. Because the only way for a family to retain their pure-blood status is to marry other pure-bloods, all pure-blood families were related to one another by the 1990s. Because of inbreeding among pure-bloods and some marrying Muggles and Muggle-borns, the number of families claiming to be pure-blood is declining.

Purebloods are people who (mainly) have no Muggles in their family. They like to keep the generations "pure" by breeding with other purebloods. Purebloods are generally the ones who use the term "Mudblood", a term considered derogatory by the wizarding world. Mudblood means Muggleborn, suggesting they have dirty blood and thus have no right to be a witch/wizard. The Weasley family are considered blood traitors as they support Muggles, Muggle-borns and Half-bloods.

Maintaining "purity"
Some pure-blood families can trace their pure-blood status through many generations of magical ancestors and deny ever having any Muggles within the family, such as the House of Black. However, the truth is that if they ever did exist in the past, true pure-blood wizards and witches do not exist today. They merely erase Squibs, Muggle-borns and Muggles from their family trees.

Those who consider blood purity very important also hide their Muggle ancestry, clinging to the magical heritage they do have. Many Death Eaters are believed to have done this, and their leader himself took on the name Lord Voldemort out of a desire not to keep the name of his "filthy Muggle father". It is likely that Voldemort told his followers he was a pure-blood, given the reaction of Bellatrix Lestrange to Harry's "accusation" that he was in fact a half-blood.

Many pure-blood families, such as the Blacks and Gaunts, practice marrying cousins in order to maintain their pure-blood status, and disown any members who marry someone who is not a pure-blood. This practice has been known to cause problems as families become inbred; for example, the Gaunts showed signs of violent tendencies, mental instability, and even diminished magical capability. The number of pure-bloods is diminishing over time because of inbreeding.

It is theoretically possible for a family to "regain" pure-blood status after a time. For example, although Harry Potter's mother Lily was Muggle-born, thereby making Harry a half-blood, his wife Ginny was a pure-blood. If each of their three children were to marry pure-bloods, any children they had could be considered "pure-blood" as well, as they would have no Muggles or Muggle-borns among their grandparents.

Attitude
Some pure-bloods regard themselves as superior to those witches and wizards who are born to Muggles and half-bloods. They believe that Muggles are little more than animals, and that Muggle-borns, whom they derisively call "Mudbloods", are second-class citizens who are inferior and unworthy of being allowed to practice magic. Many also discriminate against half-breeds, werewolves, and magical creatures such as Centaurs.

Salazar Slytherin believed that only pure-bloods should be allowed to attend Hogwarts School of Witchcraft and Wizardry and educated in magic; this was the cause of his conflict with the other Founders and his leaving the school. However, he left behind the Chamber of Secrets in the hope that one day his heir would open the Chamber and release the basilisk within to target Muggle-born students. Tom Marvolo Riddle was this heir, and the basilisk petrified several Muggle-born students in 1943 and the 1992–1993 school year and managed to kill one student, Myrtle, in 1943.

Tom Riddle, later known as Lord Voldemort, continued his persecution of those who were not pure-bloods (ironically being a half-blood himself) with his followers, the Death Eaters. They strived to place pure-bloods in control of the wizarding world and to rid it of Muggle-borns. When Voldemort succeeded in taking over the Ministry of Magic during the height of the Second Wizarding War, his followers created the Muggle-Born Registration Commission in order to weed Muggle-borns out of the population, and send them to Azkaban for allegedly stealing magic. Pure-bloods were also blatantly favoured within the Ministry and Hogwarts School of Witchcraft and Wizardry.

However, there are pure-bloods who do not have these beliefs, and instead treat all magical people and Muggles as equals. These pure-blood folk are often labelled "blood traitors" by the more fanatical pure-bloods.

Other

 * Pansy Parkinson and Blaise Zabini are likely pure-bloods, or at least claim to be based on the attitudes they express towards "mudbloods"and "blood traitors", and their affiliation with the prejudiced Draco Malfoy.

Author's comments
On the subject of blood purity, J. K. Rowling has said the following:


 * The expressions ‘pure-blood’, ‘half-blood’ and ‘Muggle-born’ have been coined by people to whom these distinctions matter, and express their originators’ prejudice. As far as somebody like Lucius Malfoy is concerned, for instance, a Muggle-born is as ‘bad’ as a Muggle. Therefore Harry would be considered only ‘half’ wizard, because of his maternal grandparents. If you think this is far-fetched, look at some of the real charts the Nazis used to show what constituted ‘Aryan’ or ‘Jewish’ blood...the Nazis used precisely the same warped logic as the Death Eaters. A single Jewish grandparent 'polluted' the blood, according to their propaganda.

This statement may indicate that all of a wizard or witch's grandparents must have magical heritage — i.e. that he or she cannot have a single Muggle or Muggle-born grandparent — in order for him or her to be considered pure-blood. The detailed and lengthy family trees maintained by pure-blood families such as the House of Black suggests that more than two generations of magical heritage are required to be considered pure-blood. However, this may only be the standard applied by extremists.