Tendinous intersection

Overview
The Rectus abdominis is crossed by fibrous bands, three in number, which are named the tendinous inscriptions (or tendinous intersections); one is usually situated opposite the umbilicus, one at the extremity of the xiphoid process, and the third about midway between the xiphoid process and the umbilicus.

These inscriptions pass transversely or obliquely across the muscle in a zigzag course; they rarely extend completely through its substance and may pass only halfway across it; they are intimately adherent in front to the sheath of the muscle.

Sometimes one or two additional inscriptions, generally incomplete, are present below the umbilicus.

Colloquial reference
If well-defined, the rectus abdominis is colloquially called a "six-pack." This is due to tendinous intersections within the muscle, usually at the level of the umbilicus (belly-button), the xiphisternum, and about halfway in-between.

An extremely well defined abdominal section can appear to be an "eight pack", as all eight sections of the abdominal muscle become defined. This definition is prominent among athletes with low body fat percentages, such as bodybuilders and swimmers.