Pure-blood is the term applied to wizards and witches who have no Muggle blood, Muggle borns, or half-bloods at all in their genealogical pedigree. Although technically pure-bloods have no Muggle ancestors, the small wizarding population means that "true" pure-bloods are rare or even non-existent, with most just ignoring or disowning the few Muggles in their family.^[2] Identified pure-blood families include the Blacks, the Lestranges, the Crouches, the Fudges, the Gaunts (though that line died out before the beginning of Book 1), the Longbottoms, the Malfoys, the Potters and the Weasleys. To maintain their blood purity, supremacist families have been known to inbreed into their own families by marrying their cousins, resulting in mental instability and violent natures.^[3][4] Over the course of the books, some of the remaining families die out, while others find themselves on the brink of extinction with only one male heir, such as the Malfoys, who seem to have no one but Draco Malfoy. Some, such as the Lestranges, do not seem to have a heir (though Bellatrix and her husband Rodolphus are known not to have children,^[5] and it is not impossible that Rastaban or other members of the Lestrange family have children; however, no Lestrange student is mentioned at Hogwarts). With the death of Sirius, the only surviving member of the Black family is Narcissa Malfoy. By the conclusion of the series, the Weasley family is the only known pure-blood family to have several male heirs.

Pure-blood supremacists believe blood purity is a measure of a wizard's magical ability – notwithstanding examples of highly skilled Muggle-born witches like Hermione Granger and Lily Evans, and less skilled pure-bloods such as Neville Longbottom – and Muggles to be low-life, having no magic in them. Supremacists apply the term "blood traitor" to pure-bloods who harbor no prejudice against non-pure-bloods (enjoying their presence and relations with them).

The antagonistic wizards in the Harry Potter books are almost all supremacists, while Harry and his friends disagree with this ideology. Rowling draws several parallels between the pure-blood supremacists and Nazi ideology in Harry Potter and the Deathly Hallows (the belief that pure-blood wizards have the right to subjugate the Muggle world and view themselves as a "master race", laws requiring Muggle-borns to register with the Ministry of Magic, rounding up Undesirables, etc.).^[6]

Not all pure-blood wizards are advocates of pure-blood supremacy: the Weasleys and Longbottoms are old pure-blood families, but no known members of these families are sympathetic to supremacist aims.^[3][7][8] The Black family, traditionally pure-blood supremacists, also seem to have produced one or two such "black sheep" in every generation, namely Sirius and Andromeda (Bellatrix and Narcissa's sister who married the Muggle-Born Ted Tonks).

Several wizards question the notion of blood purity altogether. In The Tales of Beedle the Bard, Dumbledore asserts that the much-vaunted blood purity does not exist, and is only a fiction maintained by the deceptions of supremacist wizards.

- Purebloods, The Harry Potter Universe, Wikipedia