Views on the History of 'Pure-blood Wizards'
by Albus Dumbledore
There are two different definitions of 'pure-blood wizards': the popular social definition, and the esoteric, magical definition. The magical definition of 'pure-blood wizards' of course, predates the popular definition by several centuries.
The concept of 'pureblood' originated with blood-based magic. Certain types of magic, particularly in the Dark Arts, require the use of human blood. Those texts are quite clear that the blood used in these ceremonies be 'pureblood.' It isn't entirely certain that this specifically refers to pure-blood wizards (as opposed to half-human races), or if those participating in the ritual must be pure of intent, or even if it means one must have a familial or 'blood' connection. However, few have attempted to test the current definition - that the blood spilled must be of wizarding descent on both collateral lines - given the likely consequences if they are wrong.
The popular social definition of 'pureblood' began in the 'Dark Times,' (known to Muggles as the Middle Ages). In the 'Dark Times,' when wizards and witches were hunted assiduously by Muggles, the magical aristocracy understandably seized on these spells as proof of their superiority over other species, over 'half-blood' wizards, and especially, of their superiority over the very Muggles who were hunting them like animals. They proudly traced their lineages back many generations, believing that a 'purer' lineage resulted in purer, more powerful wizards. The fact this has never been proved - indeed it has been disproved often - has done nothing to dispel this popular myth.
Thus, the social definition of 'pureblood' is firmly entrenched with most wizards to mean one who can trace back his ancestry through many generations of wizards on both sides of the family, although there are squabbles over exactly how many generations this must be. In a remarkable coincidence, wizarding families who claim to be 'pureblood' tend to adopt the definition that requires the exact number of generations as their own family, or slightly fewer.
The true (or at least current) magical definition of 'pure-blood' wizard, on the other hand, requires only that both parents of said 'pure-blood wizard' be either a wizard or a witch (and not half-human or Muggle).
One must note that for all the fuss over the subject, spells that specifically require the blood of pure-blood wizards are quite rare, although naturally the pure-blood aristocracy claim that any spell requiring any blood is best done with pure wizarding blood. The fact that many of those rare blood-magic spells require the death of the wizard or witch casting them, or the death of the pure-blood wizard themselves, or a fundamental alchemy within the pure-blood wizards participating, thus making them more or less an ingredient and not exalted at all, tends to be ignored….
Excerpt from 'Ramblings on Various Historical Facts of Interest to Me, If No One Else' (currently sold out).