Use the noun voracity when you're describing someone's enormous, gluttonous appetite.
Some people eat a little and others eat a lot. The ones who eat an extreme amount have the quality of voracity — basically, it means overeating. Sometimes this word refers to literally eating too much, and other times it refers to the desire to overeat. Anyone might eat a couple of hot dogs, but only someone with voracity could eat ten or eleven in one sitting. At the root of voracity is the Latin word vorare, which means "to devour."