Hi Yijaha - they are more or less the same, interchangeable. Stomach is the more formal word. I'm not sure how these words would be used to describe a gym exercise, but stomach is probably the best to use. You exercise your abdominal muscles (or "abs", as in "ab crunches"), which you could also call (less technically) stomach muscles or tummy muscles (informal). You would probably not use "belly" when describing a gym exercise.