WebMD: “Ketogenic” is a term for a low-carb diet (like the Atkins diet). The idea is for you to get more calories from protein and fat and less from carbohydrates. You cut back most on the carbs that are easy to digest, like sugar, soda, pastries, and white bread.
Wikipedia: The ketogenic diet is a high-fat, adequate-protein, low-carbohydrate diet that in medicine is used primarily to treat difficult-to-control epilepsy in children. The diet forces the body to burn fats rather than carbohydrates.