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. 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. The Gaunts displayed both tendencies by the time of the sixth book with the exception of Merope, the last female member of the line.
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).