The terms I've heard used to describe the gap between abs are cut and groove. You might, for example, be striving for deeply cut or well-grooved abs.
Here are two random examples that use these terms. From bodybuilding.com:
We all strive for those etched in stone abs with deep cuts that you could wash clothes on. Get great ab building information right here.
From a bodybuilding guide on streetdirectory.com:
This approach of size enlargement will help you increase groove depth in between each pack once you have already been in fat burning workout routine.
A general term for a deep narrow line is furrow:
Noun:
- A trench cut in the soil, as when plowed in order to plant a crop.
Don't walk across that deep furrow in the field.
2. A deep wrinkle in the skin of the face, especially on someone's forehead.
When she was tired, a deep furrow appeared on her forehead.
Verb:
- To make (a) groove, a cut(s) in (the ground etc.).
Cart wheels can furrow roads.