The term 'belly muscles' refers to the two main muscles of the abdomen, the rectus abdominis (also known as the 'six-pack muscle') and the transverse abdominis. The rectus abdominis is a long, flat muscle that runs from the rib cage to the pubic bone and is responsible for flexing the spine. The transverse abdominis is a deep muscle that wraps around the torso and helps with stability and posture. Both muscles are important for core strength and stability.