The terms I've 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:

1. 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:

1. To make (a) groove, a cut(s) in (the ground etc.).
   Cart wheels can furrow roads.

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:

1. 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:

1. To make (a) groove, a cut(s) in (the ground etc.).
   Cart wheels can furrow roads.