when people don't leave their houses for days or hours (depending on how fast a reader one is) at a time as a result of the latest harry potter book coming out. nobody goes online or outside for fear that someone might reveal the ending.
girl: Mom, can I hang out with you tonight? The new Harry Potter just came out and nobody's free. On second thought, maybe I'll just go out and buy the book.
When something ubearable and disgusting becomes extremely popular only because it's... popular.
This usally happens when the popular kids at some schools take interest in something (those kids are dumbasses and have no sence of judgement) stumble across something they see on MTV or something and fall in love with it. The popularity will spread through America (and sometimes the world, like this definition's name sake: Harry Potter). The people's minds will be convinced that they like what they are seeing/hearing/reading/watching. If these objects were not popular, anybody who would have been a fan would notice how horrible it really is.
Harry Potter, Mad TV, Avril Lavigne, Lil Jon and the Eastside Boys, Ashlee Simpson, Hillary Duff, and Dragon Ball Z are all products of the Potter Effect.