I felt it was high time someone made a definition without opinions so here goes. Nu metal is a subgenre of metal emerging in the late 90s with bands like Korn,hence the name "new metal" later changed to nu metal to define it late ron when it wasn't new anymore. Nu metal fuses several subgenres of metal(mostly death and thrash metal)with rap VERY different from rapcore(rap metal)which is simply hiphop with metal as musical backing. Notice slipknot's use of turntables another nu metal thing,heavy use of turntables and such. No nu metal band is without a keyboard and synthesizer.
A perfect example of nu metal is linkin park's faint, notice how some of the lyrics are sort of rapped but others are sung in the traditional metal way (high pitched yelling, grunts and growls etc.)
Nu metal contrary to popular belief is real metal just new...anyone who was a metalhead in the 80s probably dislikes nu metal simply because its new. Face it, adults have always frowned at their kids music. Furthermore several nu metal bands have played at ozzfest before(and who are we to question the judgement of lord ozzy?).
Slipknot(sort of), Korn, Limb Bizkit, Linkin Park (exluding minutes to midnight). Nu metal.