Might suggest treating the ab like any other muscle group. The ab structure is very strong (but not large) by nature and doing marathon workouts, almost daily, like a lot of people do may not be that advisable.
Three times a week, or even two, may be a better options. Keeping the reps between 8-12 and 4 to 6 sets total for the ab muscle structure can be better. Thick ab's can be very impressive, as long as the body fat ratio is kept to a respectable level. If your ab's have the fusing together look, than their muscle mass is probably breaking down and not building up.
If your gym has a real Roman Chair (like the old, old timers used back in the day), and used the correct way, than that can be one of the better mid-section exercises around. You can also use heavier weight with most any ab work, so feel free to dso that, for more defined ab's. Good Luck.