Other forms: undercooked; undercooking; undercooks
If you undercook food, you don't heat it all the way through. It will be either too raw to be safely eaten or not cooked enough to bring out its best flavor or texture.
Undercook the Thanksgiving turkey, and your family will likely never let you forget it! But if you do undercook the turkey, don't serve it! It will not be safe to eat. No matter how cold the mashed potatoes are getting or how cranky Uncle Oshkosh is being, put that bird back in the oven until the meat thermometer says it's done. It's better to make everyone wait for dinner and eat reheated mashed potatoes than to send all your guests home sick!