"Do you know about the principle of contamination?"
"Leftovers, right?"— Map of the World, ch. 5
Contamination is a principle most relevant to magical practices. In essence, it is the idea that everything leaves behind a trace of itself. It is first introduced in Map of the World, but plays a role across all books. If contamination is deep enough, it may allow one to lay claim to something (or someone) by right of ownership, though it has little to no meaning on this plane of existence.
Contamination, most frequently, becomes noticeable in its negative forms. It can be removed, at least from inanimate objects, through cleansing rituals.
It's in everything to a greater or lesser degree, but generally, it's only noticeable when it's negative, and in those cases dramatically so: poltergeists, coercion spells, compulsions, summoning the more disreputable supernatural beings for various reasons, ritual human sacrifice.— Map of the World, ch.5
Examples of Contamination
"Demon blood fed to a kid," Dean says quietly. "Contamination."— Map of the World, ch. 5
Castiel and Dean Winchester
In rebuilding Dean's body after hell, Castiel contaminated every cell of it with himself. This allows Castiel to construct his "cabin of invisibility".[1.1]
- Map of the World
- ch. 5