If you are female, the odds of seeing abdominal muscle definition are stacked against you since you naturally have a thicker layer of sub-dermal adipose tissue. That is normal and healthy for a woman.
I spent a lot of years working out my abs trying to get an awesome six pack with no visible results, and I actually discovered the "secret" years later quite by accident.
I'm not in the habit of taking shirtless pictures of myself, but in my line of work you'll find a bunch of them posted on the internet, so here's a photo object lesson about abs:
1. My first professional kickboxing match back in 2004: Weight 170 lbs, body fat percentage around 13%
2. Here's a picture from 2007: Weight 155 lbs, body fat percentage 3.1%
3. And here's a picture at a K-1 tournament a couple of weeks ago: Weight 175 lbs, body fat percentage about 10 percent
A lower body fat percentage is not necessarily a good thing, unless you are a professional athlete- and even then professional athletes should only maintain low a BF% for a short time. 3.1% is exceptionally low, and potentially dangerous. The average healthy man has a BF% of 18% and the average healthy woman has a BF% of 21%.
So what did I actually did to drop to that ridiculously low 3.1% body fat and get those killer abs back in 2007?
Intense abdominal exercises? Nope.
Yoga & Pilates classes? Nope.
A super intense MMA training camp? Nope.
I did 2 things at the same time:
1. I spent a year working at a factory 12-14 hours a day consistently lifting objects ranging from 50-100lbs. Essentially, I did hundreds of Olympic lifts every day (Squats, cleans, deadlifts, & snatches) using oddly shaped objects: boxes, trays, vats, machinery, etc.
2. I ate every meal at the employee cafeteria (we had a good one fortunately) which amounted to controlled portions of high-volume, low-fat whole-foods, and a very structured time frame to eat 3 square meals a day with 2 breaks for snacks. (I actually ate around 4000 calories a day, the average is around 2000 - so please don't think I'm telling anyone to starve themselves. If you starve yourself, you will not get fit. It's as simple as that. You need to eat good food, and enough of it if you want to be healthy and look healthy)
Honestly, I was just trying to eek out a living- getting a six pack didn't even cross my mind at the time.
After studying health & fitness for several years, I finally figured out what was going on, and it's not rocket science:
If you want to burn fat, you must build muscle. If you want to build muscle, (1.) you need to work out the biggest muscle groups in the body. (2.) You need to feed those muscles with some quality food.
You can't build any significant amount of muscle by doing abdominal exercises. Abs are puny compared to the muscles in the legs, butt, back, and chest. That's why Olympic lifts consistently worked to change my shape and doing thousands of situps and crunches didn't. Squats, deadlifts, cleans, and snatches use the biggest groups of muscles in the body in combination. It's like killing two birds with one stone. You need to build more muscle, so you've got to work more muscles at once.
The human body uses fat stores primarily at night during sleep. If you have a high percentage of lean muscle, you will burn a high percentage of body fat during sleep. If you have a low percentage of lean muscle, it doesn't matter how strong your abs are, you won't burn up much fat while you sleep.
Most mammals use fat stores the same way. That's why bears spend all summer running around, eating and getting fat before sleeping all winter long. When they wake up from hibernation, bears aren't fat anymore. Their bodies eat up all the fat while they sleep.
Anyway, you want abs that show? In short, stop focusing on the abs. Change your diet to quality whole foods (Do not starve yourself!) Change your exercise routine to whole body strength training. Don't get me wrong, I love abdominal exercises for functional fitness, I do them all the time, I teach them- but you'll never hear me promising a six pack if all you do is work your midsection.