Is there a medical term for the groove or border between developed muscle groups, such as the abdominals’s “sixpack”
The “sixpack” is a single muscle, the rectus abdominis, and the grooves are called the tendinous intersections, a term unique to that muscle. Most other such “grooves” are just the space between muscles, and while I think some folks call them intersections, I’m not aware of any universally accepted term.
The fibrous band between the right and left recti abdominis is known as the linea alba. Each lateral border is known as the linea semilunaris. Although these are fibrous structures connecting muscle, their positions would be visible in a person with a six-pack.
I think the word you are looking for is striations.