It's the name that the wretches call the people in the Dome. Next!
Okay, so the title Pure might seem pretty obvious at first, but let's not get carried away here. Throughout the novel, the word "pure," and even the definition of the word "pure," is crucial to our interpretation. For example, Partridge is referred to as "The Pure," when he escapes to the outside of the Dome. But is Partridge really a pure person? Er, no.
Partridge can be extremely frustrating; he couldn't even figure out that the swan wife song was a riddle. (Come on buddy.) He's also exists in a moral gray area: he hates his father, he seeks revenge, and he's impetuous. Just because he has clear skin and is genetically altered, does that make him pure? That's for the reader to decide as she follow's Partridge's story.
This title is also meant to be thought provoking. This novel makes you ruminate on the nature of purity. What is purity, exactly? Are the Pures really pure? And are the hideous wretches outside of the Dome really wretches, or do you think that they might be purer than those who live inside the Dome?