They are interchangeable, but there are differences. "Stomach" is the organ in charge of processing the food, being this word the most formal and technical. "Tummy" is a nice and childish way to call the stomach, mostly used by kids and their parents. "Abs" are the muscles around abdominal area, which cover the stomach inside. "Belly" is the exterior area in the body where the stomach and the abs are located. Normally, if you exercise your abs, take care of your nutrition and burn all the fat in the abdominal area, your stomach is little and you don't have a belly.

"Stomach" means the internal organ where food goes after you swallow it. It is both an everyday word and a proper medical word. "Belly" means the whole general area around the navel, the soft area that sometimes bulges a little, and everything inside. The proper medical word is "abdomen." It is not the same thing as "stomach." A pregnant woman has a large belly. The baby is, of course, not in her stomach, it is in her uterus. However, the baby is "in her belly." "Tummy" is an informal word. It can mean "stomach" or "belly," either one. It is a little bit rude to discuss internal organs and body processes. "Tummy" is a "nursery word." We use it with children. If you think of the way the word sounds, you will realize that a young child learning to talk might say "Tummy" because "Stomach" is too difficult. The diminutive ending make it sound cute or playful. In US culture, if you are at an amusement park and someone says "Would you like to get some fried dough," you would not say "No, thank you, my stomach hurts and it would make me vomit." You would say "No thanks, my tummy isn't feeling quite right." "Abs" is a word used by athletes and body builders. It is very specific. It is an abbreviation of the phrase "abdominal muscles." The abs are the muscles under the skin. They are outside all of the organs in the abdomen. I am not very fit and my abs are not developed. So, my belly is smooth and rounded, due to the layer of fat I have. A person who works out will "develop" his or her abs. The muscles get bigger, the layer of fat gets smaller. You can start to see the outline of the muscles in the shape of the abdomen. Our cultural ideal of a fit person is someone with a tight, flat belly and the visible shape of the abs. Originally, the phrase "six-pack" meant a package of six cans of beer. A set of well-developed abs shows up as six bulges, and nowadays this, too, is informally called "a six-pack."

belly, stomach, tummy - these are interchangeable abs refer to the core muscles, which are separate