Aromatics (aromats): Abbreviated in kitchens as aromats, aromatics refers to aromatic vegetables and herbs, commonly onions, carrots and celery (aka mirepoix), thyme, parsley, bay leaves, garlic and peppercorns, or lemongrass, ginger and scallions in Eastern cuisine. Aromats may include anything added to a stock or a stew to enhance its flavor (though not everything is aromatic or should be used as an aromat—bell peppers, turnips and zucchini are not aromats). We associate aromats with sweet and floral flavors, and they’re usually removed from the finished preparation once they’ve imparted their flavors. Abundant use of aromats makes an enormous difference in the end result and their importance would be difficult to overstate. Their use is not relegated to stocks and stews; sauté mushrooms in oil in a hot pan, then try sautéing mushrooms in oil in a hot pan to which a few stems of thyme and garlic cloves have been added and you will taste the impact of aromats.
Mirepoix traditionally means two parts onion, one part carrot, and one part celery, roughly chopped, as you see above, in about that ratio (celery is optional; many avoid it on purpose; pay attention to the flavor and make your own call). This mixture adds sweetness and aroma to a stock or sauce. (Mirepoix is usually strained out of the stock or sauce it flavors but if it is not, then it is called matignon.) Thyme, perhaps the best and most elegant and most versatile herb (above left), bay leaf (careful with these, their strength varies), peppercorns (I find that they don’t make a difference unless you crack them with a sauté pan), and on the far right parsley (flat leaf, which has a more delicate flavor than curly). You can transform food in a wonderful ways using these items. You can even save that can of hopeless Swanson’s chicken broth given enough them, and that’s saying something (though better to start with water). Related terms are sachet d’épices and bouquet garni, specific bundles of aromats. Pay attention to your use of aromats—they can make all the difference.
UPDATE FROM ON HIGH:
A few brief general responses to questions:
Fennel is a great aromatic vegetable and works well in fish and vegetable stocks. … Organic stock may be great, and it’s likely better than non-organic—I haven’t tasted it. … For stock, it’s fine to use a carcass that’s been brined. … It’s ok to freeze bones for later use in stock. … Roasting bones and vegetables creates more complex flavors; fish is so delicate, their bones aren’t typically roasted for stock, though there’s no reason you shouldn’t try it.
On veal stock—if you’re having trouble finding bones, ask your meat department for a veal breast, which has both meat and lots of connective tissue and is reasonably priced—have them cut it into pieces for stock.
Chlorinated water: Chlorine does dissipate quickly (which is why dogs drink out of toilets—they don't like the fresh chlorinated stuff in their bowl). In my opinion, tap water should always be used for stock unless it's unpleasant to drink.
Is a turnip an aromat? What an interesting question. I'm with Dervin, if a turnip and a green pepper can be considered aromats, than ANYTHING can be, and the word ceases to have meaning. When do we ever add a green pepper to sauce intending to strain it out after it's imparted its "aromatic" effect. Perhaps that should be part of the definition, a veg or herb that we intend to remove before it's ultimately served.