Tummy is an informal shorthand for stomach. This word is mainly used for talking to little kids: "Aw, do you have a tummy ache?"
You wouldn't use the word tummy in your human anatomy class, but you might use it when you babysit your two-year-old niece. Infants sometimes have "tummy time," a short period when they are turned onto their stomachs so they can develop their neck and shoulder muscles. Tummy has been in use since the 19th century, and experts guess that it comes from a childish pronunciation of stomach.