Little ball created out of bits of your cotton T-shirts that you find in your belly button. It was mentioned in one episode of Ren & Stimpy, in which Stimpy enters his belly button, where he meets a Belly Button Elf who serves him lint loaf as his dinner.
Woah! Do you EVER remove that lint loaf?? It's starting to develop a mind of its own, plus it smells like crap.