I think 'abdominal furrows' is a nice way to say it.
If you examine the back of your hand, you may see what could be phrased, "venous furrows". If you then extend your fingers, you may see what could be phrased 'tendinous furrows'.
Look where your thumb and wrist meet, extend your thumb, and you will see the 'anatomical snuffbox', it too is a furrow.
The 'six pack' or 'washboard' forms zig-zagging furrows over the abdominal region.
This is different from folds of the skin which are called 'creases'. For example, look at the palm of your hand. These folds of the skin are what the palm reader interprets.
Wrinkles are a type of crease meant to point out folds on the skin due to age.
Creases are folds pushing the skin closer and are often jagged, whereas furrows make a smoother valley. Although, the forehead during an expression of surprise is an example of compressed furrows.