1. Anat., Zool. a. a saclike enlargement of the alimentary canal, as in humans and certain animals, forming an organ for storing, diluting, and digesting food. b. such an organ or an analogous portion of the alimentary canal when divided into two or more sections or parts. c. any one of these sections. 2. Zool. any analogous digestive cavity or t
Generally, "tummy" is what you say to children under 10 years old or so. It's a comical word when used between adults. You can use "tummy" with adults, but it will sound childish, but if the context is serious, always use "stomach".
Btw, the word "belly" or "gut" is often used for the stomach informally, although both generally refer to the entire abdomen (from the bottom of the ribs to