From urban dictonary
Full name: Renesmee Carlie Cullen
A half-vampire spawned by the famous dynsfunctional teenager lovers, human Bella and vampire Edward Cullen in the fourth book, Breaking Dawn, of the Twilight Saga written by Stephenie Meyers. With bubbly brown eyes and a need to suck your blood, who can resist this little leech? Not Jacob, who upon first site on this child, falls madly in love. Though rest assure, he's not a pedaphile. He's giving her a promise ring. In about seven years, Renesmee will already reach adulthood (eighteen years) due to her quick growth and maturity. She also possesses a gift much akin to telepathy, letting her mother and father communicate with her even when she cannot speak yet. It is assumed her future will arise to greatness, and also some mad lovin' with Jacob.
When giving labor, Bella was about to die. As a human still, she did not have enough strength to endure the little creature trying to squirm -cough, claw, cough- her way out. Edward, always the knight in shining (literally, he shines) armor, simply chewed his way through Bella's uterus, allowing both to live. Bella also was turned into a vampire as an effect of this, er, magnificent deed.
Later, Renesmee is thought to be a danger by the ruling vampiric family the Volturis. Bella plans to have Renesmee run away with Jacob, but thanks to Alice, Renesmee's rare predicament of being a half-mutt isn't so rare at all. Another halfing exists in good condition, proving that Renesmee will be okay. Cue happy ending.
Renesmee is also called Nessie, like the Loch Ness Monster, much to Bella's chagrin. And any other loyal fan's too.