Title pretty much tells the story: I want to tone my stomach and develop some visible abdominal muscles...I'm not going for an eight-pack or anything extreme, I just need some moves/workout ideas that would give me a decent-looking midsection. I want my stomach to be flat, but slim and in-shape looking...if that is even possible.

One word, Pilates.

Core stability exercises are the key with cardio, cardio, cardio. (Did I mention cardio?) You have to get your body fat percentage down if you are looking for definition and eat really clean.

You've really got two options.

1. If your core is strong, get lower body fat %, then the muscle will show.

2. If you're weak, get stronger (and make sure to work the transverse abdominis). There's a jillion ways to get at this, the internet is your friend. My personal favorite are: ab roller, plank variations, sprinting(yes, sprinting), hanging leg raises and vacuums. There's cool instability work you can do with exercise balls, bosu balls, foam rollers, suspension trainers(TRX) and weights. Once you get a hang of ab work it's easy to up the difficulty.

3. This is just a combination of 1 and 2.

IMO the old standard of situps and crunches isn't the way to go, not that they don't work, but that they're often done incorrectly and they don't work all the muscles that need working. Whatever exercises you choose to do, make sure you get your form right, if you mindlessly go through core work, you can hurt yourself. Make sure to keep the muscles engaged and focus on the task at hand.

Assuming that you're a dancer (this is dance.net) look into anti-rotation exercises for your core. The bird dog is an example of this, but there are plenty more. This will help you keep your body held as you turn and jump (hence the anti-rotation, aka keeping your shoulders above your hips, not twisting).

Good luck

In order to achieve visible muscles (or definition, they are both the same thing), you must lower your bodyfat %.

A defined stomach comes in at around 18-19% bodyfat. Abs come in at about 14-15% bodyfat, but this is VERY difficult for women to achieve.

Lowering your bodyfat comes by eating at a deficit first and foremost. You can't out-exercise or out-cardio what you put in your mouth.