Pureblood

"The term 'pureblood' refers to a family or individual without Muggle (non-magic) blood. The concept is generally associated with Salazar Slytherin, one of the four founders of Hogwarts School of Witchcraft and Wizardry, whose aversion to teaching anybody of Muggle parentage eventually led to a breach with his three fellow founders, and his resignation from the school."

- Where the term 'pureblood' originated from

Pureblood is the term for wizarding families who claim to have no Muggles or Muggleborns in their family tree; however, when traced back far enough, nearly every family tree had some non-magical ancestors. Most wizards and witches consider an individual to be a pureblood if his or her parents and grandparents were not Muggles or Muggleborn.

By 1995, the number of pureblood families were declining. Most pureblood families were related by then in an effort, by some, to retain their status by marrying only other pureblood families, but it had eventually led to inbreeding. While some families, such as the Malfoys, had allowed for marrying halfbloods, others such as the Gaunts, Lestranges, and Blacks, required their family to only marry pureblood individuals, or those with no Muggle or Muggleborn ancestors. This refusal of marrying Muggles or Muggle-borns had led to difficulty in propagating their families.

Overview
"The pureblood families are all interrelated. If you’re only going to let your sons and daughters marry purebloods, your choice is very limited; there are hardly any of us left."

- Sirius Black's overview of blood purity

Pureblood individuals are people who have no Muggles or Muggleborns as parents or grandparents. Traditionalist purebloods like to keep the generations "pure" by breeding with other purebloods and are generally the ones who use the term 'Mudblood', a term considered derogatory by the wizarding world. Families of pure-blood status often avoid marrying their heirs off to Muggleborns and Halfbloods, believing that the next generation could lose their capability to perform magic. Mudblood means Muggleborn, suggesting they have dirty blood and thus have no right to be a witch/wizard. Purebloods who do not share these prejudices are considered "blood traitors" by the ones that do.

In the 1930s, a wizard, commonly believed to be Cantankerus Nott, wrote the Pureblood Directory featuring twenty-eight families he believed to be truly pure-blooded, though many objected to this designation.