A genetic mutation occurring with a cat and a Pop Tart resulting in the ability to have an addicting annoying song, something to troll about, the icanhazcheeseburger franchise, and making rainbows come out of your butt while flying through the air. The rainbow poop can be turned into monochromatic poop if the Nyan Cat's diet isn't maintained. His diet is small children and bananas.
Son: "Mommy, why are we putting little Jeffery outside?"
Mom: "We are feeding Nyan Cat, dear. We don't want his rainbow poop to turn monochromatic."