The term 'belly muscle' refers to the abdominal muscles, which are a group of muscles located in the front and sides of the abdomen. The most commonly known abdominal muscle is the rectus abdominis, which is often referred to as the 'abs'. This muscle is responsible for flexing the spine, and is often used in exercises such as sit-ups and crunches. The other abdominal muscles include the external obliques, internal obliques, and transverse abdominis. These muscles work together to provide stability and support to the trunk, and help with posture and balance.