The term 'six-pack muscles, for short' is an abbreviation for the abdominal muscles, which are commonly referred to as the 'abs'. The abs are a group of four paired muscles located in the front and sides of the abdomen. These muscles are responsible for flexing the spine, aiding in breathing, and stabilizing the core. They are also important for posture and balance. The term 'six-pack' is derived from the visible definition of the abdominal muscles when they are toned and well-defined.